Yay! Great to see this!
Audio doesn't work on one of my six Foscam FI8918W units, and I believe it's just defective hardware.
Audio on all units (of those that work) exhibits loud mechanical background noise. The mic seems quite sensitive, though. Sounds in the room do overpower the background. Automatic gain control by the camera is perhaps involved: when no sound is detected, gain is amped up and produces the noise.
If you need iPhone->Foscam support right now, try Livecams Pro (for iOS at least). (I much prefer iCam but Livecams Pro supports a few extra functions that come in handy.) Livecams Pro exhibits the same background noise, so the background wouldn't seem to be anything that can be fixed on the Mac/PC/smartphone side.
Audio delay in either direction (cam->phone, phone->cam) is about 2 seconds over my LAN.