Is there a standard out there for audio over WiFi. Bluetooth has that standard (A2DP), but not sure if WiFi has it. If not, then somone will have to create it, and that may be why there are no apps yet on the market.
ALso, the EVO is probably more prone to sending audio over WiFi or Bluetooth. Not that it has to be, but sometimes manufacturers can't see past their own audacity, and it could be that the EVO never was deisgned to capture and play audio from any of the radios...rather, it's role is to send audio over those radios. Again, this is speculation.
You'd be happiest with bluetooth headphones, and a bluetooth dongle for the laptop (cutting the EVO out of the equation).
Can the EVO act as a Bluetooth headset? If so, then you don't need the headphones. But as mentioned, did HTC ever envision the EVO being the headset ("slave") in any given bluetooth pairing? I've honestly never tried. Can you have a bluetooth phone discover and setup the EVO as a bluetooth headset? If you can do this, then you can have what you want (only over bluetooth, not WiFi)