For me, the soundtrack is one of the most important things to consider when making short films. There are tons of great songs that I'd love to use. Unfortunately, the licensing fees required to use them legally are out of my price range. Like photography, music makes money via licensing. I respect this since my business operates a similar way. For my videos, I tend to look for music more in my budget and then acquire the usage rights.
One option is to use lower-priced music stock sites. I've spent hours listening to music from numerous sites and so far I haven't found anything that inspires me. Another option that I've found works really well is to look for smaller, local artists via myspace.com. When I find a song that I like I contact the artist and ask for the rights to use their song with one of my movies. This use is typically limited to the life of that video, regardless of the media used to show it (such as DVD, web or presentation). This arrangement is appealing because my videos are for promotional usenot for resale. In exchange for the rights to use the song I offer to photograph the band as payment. So far it has been an appealing strategy for everyone involved.
About a year ago, I used The Brokenmusicbox's song with my Macworld cover creation video. They recently decided how they wanted to be photographed: in their living room surrounded by all their musical instruments. For the final photo I gave them two options. The first was the version that I photographed using my Canon. The second version was the same photo but enhanced with the Picture Show app on the iPhone. I did that because when they came by my studio to talk about the photo shoot, they saw some of my photos that I had taken with my iPhone. Of course the final file size isn't as big, but it'll work well for onscreen use.
It never ceases to amaze me how the tone and feel of a movie can change with different soundtracks. Going through this process is actually really fun for me. I enjoy finding new bands and musicians that I hadn't heard before.