As you've probably noticed, most apps seem to be oriented toward pulling music from the PC, or remote-controlling your PC's media player, instead letting your push it from the PC to your phone in order to make a wireless connection from your PC to your stereo (which is what I think you're looking for).
Of course, that would require a 2-part solution; a PC-based media player which can stream media (like the excellent, open-source VLC Media Player) onto a network, and an Android client which can tune into the stream.
I think that when the official open-source VLC Android app (from VideoLan.org) is released in a couple months, it will have the ability to stream from the VLC Media Player on your PC exactly like you want. (The VLC Media Player can currently act as a client or server for streaming.) In the meantime, I noticed that there are two free apps on the market with VLC in their name (though not officially from VideoLan.org) that seem to accomplish what you want (VLC Remote, and VLC Stream & Convert). I haven't tried either.
Please post back if you find a great solution.