Google music can stream your music via wifi. I've played with Zumocast and i believe it streams via wifi also. Zumocast was actually nice but i ran into an issue because my movie files were not supported. I didnt feel like finding a work around or reformatting 135 movies.
If you're *not* looking for an app-hunting adventure, I Just installed motocast the other day and it does the job so far. The interface has the same function-over-form sensibility of most of moto's proprietary software (that I've seen on this one phone...), but it functions.