How much of a copyrighted MP3 file can you post on a public web page without permission from the copyright owner?
Legally, none, not one little bit. Yes, technically, none. Or at
least none of the copyrighted parts (new creative works).
But the copyright owner still has the burden of proof on a number
of issues, so a relatively minor infringement isn't going to
justify calling out the bus-load of litigators. You may get a
somewhat pointed letter that asks you what you think you're doing
(or the like) and you should take that as a sign they're warming up
the bus. Better safe than sorry, unless you have a lot of time and
money to go defend yourself in federal court.
Answer Usually, if 30 seconds or less of a song is used by a
website or business, it is considered "sampling" and not copyright
infringement. For example, Internet and Satellite radio services
must pay full royalties to the copyright owners whenever they play
a complete song. However, the songs they play as quick promos or
clips are not subject to full copyright (and may not have to pay
any royalties, depending on the terms of their license). This is
why it's legal for I-Tunes to let you listen to 30 seconds of each
song before you download it. To be safe, I would only post 30
seconds or less of a song on a public website. It is exceedingly
unlikely that the recording companies would come after your for
using "sampling" on a website.