Is there a way to stream all sound from an Android phone to a PC to be played by the boxes of the PC? At best wirelessly?
Based on some quick testing I did on my Nexus One running Android 2.2 (Froyo), it seems like all sound is routed through headphones when they are plugged in.
You could attach a male-to-male 1/8" (3.5mm) audio cable from the headphone jack on your phone (or the mini-USB-to-headphone-jack adapter, depending on your phone model) to the Line In jack on your PC. You may need to do some configuration on your PC to get sound from the Line In to play to the speakers — I recommend sister site Super User if you need help with that.
There's one caveat: although all sound is routed through the headphone jack, some sounds are still also heard through the phone's speakers. The two most notable of these are the ringer and the camera shutter sound. Depending on what exactly you're going for, that could be an issue.