Music

secular music
short answer - enjoy it.
much of the modern church holds that secular music should not be enjoyed by the christian, either because of the association, approval, or influence. personally, i don't think that any of these arguments hold up on a global basis; influence is applicable on a personal basis (rom. 14:23, mat. 5:29,30. really all of romans 14 could be applicable...), but no more than any other possible influence.

common arguments against secular music

association -
association arguments follow the idea that because the music is produced by those serving satan, christians should not listen to it. honestly, in my opinion, the reasons given for avoiding association are always one of the next two (approval or influence), so there isn't much of a point here.
basically, arguments of association run aground on the example of christ. associating with sinners/ non-christians is explicitly done in the bible by jesus, paul, and others (read any gospel or acts).

approval -
the argument here is that listening/enjoying secular music is, or may be percieved as, agreement with the non-christian origin or message of the song.
in my opinion, this is the reason most christians (who have trouble with secular music) hold. fundamentally, it seems wrong to many people to appreciate/enjoy something which conveys a message that is diametrically opposed to their beliefs (which much of current secular music is to christianity). i think that this is misleading however: appreciation does not imply agreement.
it should be noted that there is no biblical command to not appreciate/enjoy something that communicates ideas you disagree with; the argument should end with the presence or lack of such a command. however, i think that examples can be given to illustrate the point that appreciating is not the same thing as agreeing. music with lyrics that are anti-christian can be enjoyed on the merits of the melody, harmony, etc., not withstanding the idea that one can appreciate the structure, meter, and form of the words themselves though the ideas they express may be unacceptable to the christian belief structure.
the idea that enjoyment constitues endorsement also prevents christians from enjoying the creative expression of secular society. again, there is no biblical argument linking enjoyment to endorsement, but its a common feeling. in my opinion, this seems to be the same issue as whether or not a christian can enjoy the company of a non-christian; by enjoying the company of the non-christian, is the christian endorsing the non-christian's 'enmity with god'? the answer here is a clear no, since the bible clearly encourages such relationships (the gospels are full of examples and admonishments).

influence -
questions of influence invariably come down to case by case decisions; what causes one person to sin might not cause another. jesus was without sin at the wedding at cana (john 2), but a former alchoholic might have had a more difficult time (i'm not implying that drinking is a sin, but that a pattern of drunkeness is). the guiding principle is not to do anything that might lead oneself to sin; flee temptation.

demonic/spiritual influences -
some christians beleve that any music with a sufficiently loud beat is a channel for demons to curse the listener. there is no biblical argument that holds water for this view, but there are plenty of scary stories swapped in youth groups about missionaries who played 'christian' rock music only to have the animistic natives approach the missionaries and ask why they were playing the music which calls demons (the idea being that the natives used petra to place black magic curses, an idea which sounds believable to my ears). as i'm not aware of any biblical arguments along these lines, i'll let this rest.

music in the church
short answer : music is a great way to encourage and uplift the church, but it is currently often used in a manner that reinforces mystical and extrabiblical tendencies.
the first thing to establish is that the new testament recommends music/singing as a method of teaching, admonishing, and encouraging one another. col 3:16 (and eph. 5:19) states, "let the word of christ richly dwell within you, with all wisdom teaching and admonishing one another with psalms and hymns and spiritual songs, singing with thankfulness in your hearts to God." i use the word 'recommends' instead of 'commands' because the tone of the verse and passage seems to indicate that paul is presenting music as a possible (and for them, recommended) means of 'teaching and admonish one another'. historically, music has been used by the church for teaching and encouragement since its start; eph 5:19,20 speaks of music as a social/corporal tool (col 3:16 is a parallel passage) by which we encourage one another and offer thanks to god.

eph 5:19 speaking to one another in psalms and hymns and spiritual songs, singing and making melody with your heart to the Lord; eph 5:20 always giving thanks for all things in the name of our Lord Jesus Christ to God, even the Father;

the most prominent use of music in the bible is the psalms, songs whose common denominator is difficult to establish past a common desire to express biblical thought. the psalms include expression of the full range of emotions, some seemingly intended as expression to god, some for corporate use (expression to men). psalms are perhaps best understood as poetic and artistic christian expression, at times being musical prayers and at others musical teachings.

revelation also speaks of music as a directed towards god directly in heaven, perhaps the strongest argument available for the importance of music.

Rev 5:9 And they sing a new song, saying, Worthy art thou to take the book, and to open the seals thereof: for thou was slain, and didst purchase unto God with thy blood [men] of every tribe, and tongue, and people, and nation,
Rev 14:3 and they sing as it were a new song before the throne, and before the four living creatures and the elders: and no man could learn the song save the hundred and forty and four thousand, [even] they that had been purchased out of the earth.
Rev 15:3 And they sing the song of Moses the servant of God, and the song of the Lamb, saying, Great and marvellous are thy works, O Lord God, the Almighty; righteous and true are thy ways, thou King of the ages.

it should be noted that any application of the application of music in revelation must bear in mind the differences in context between the realized (heaven) and the pending (earth); in other words, if we are to build a theological ground for the use of music in the church, grounding those ideas in the situations which directly parallel our own (eph 5, col 3) makes more sense than those to which we look forward (revelataion).

the present use of music in the church unfortunately often takes a different tack all together; rather than using music as a corporate expression of encouragement, praise, and thankfulness, we use it to drive an emotional experience (one mistake being the equation of an emotional experience with a spiritual one). music becomes an essential element in christian 'spirituality' and a core part of the christian's 'communion with god', elements which do not have biblical basis. in fact, given the minor amount of time in the new testament in which music is mentioned, it is surprising that music plays such an enormous role in evangelicals concept of christianity and church. this is not to say that music is not a useful method of expressing encouragement, thankfulness, etc; but rather that we must be sure that our use of music is intended to expressly match these purposes.