Have a look on Newgrounds you can use most of the music on there for non commercial stuff as long as you credit the artist.
the non comercial tag kills that idea, because you do recive money on youtube, by adsence, so it is using a tone for comercial porpuses
however if this is not that case, you can use any background music, as long as you talk over it
edit : wait a second teaser trailer ?
do you refer fanboy movie fake trailers like ?
if so, you dont need to really consider the music, as long as you do not public add's over them, personaly i hate fanboys movie trailers it runs my day.
If you refer to an intro stage in youre movie clips well 30 seconds is enought, actually 30 seconds for an intro is a lot. you wont have any issues if "the musician doesnt speak" , you could argue you play that track youre self, however if the rock band or whatever talks, it could be an issue and your account will be denied to recieve adds, according to youtubes policy about copyright and so on, again i do not recomend to use anything, because most of the youtube channels with music are earning money and they dont like that the music is played anywhere else.
if they see your channel been "popular" they will argue is just because of the songs, but w/e
advice for any1 with a youtube channel that wants to be sponsored by machinima or some other 3rd party :
do not get adsence, let them do the work and contact you, adsence can kill your youtube channel just with missclicks on add's