When playing back .avi videos, I have noticed that the volume is really low: it is necessary to turn the TV volume all the way up and the Streamzap volume to maximum volume. Is there another control that can increase the volume level?

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R7.3: 0.22.20091023-1, Hauppauge PVR-500 (Philips FQ1236A MK4), Gigabyte Gigabyte EG45M-UD2H, E5200 2.4Ghz, 2GB RAM, NVIDIA GEFORCE 256MB

You could change the association of avi's from internal to mplayer with audio options -af or -softvol.
From an xterm try:
mplayer -af volume 10:1 file.avi
Try various volume levels till you find one that works. I tried 30:1 on a normal volume avi and it was very loud with much distortion. The :1 is for clipping and can be set to :0 and probably desirable if output to receiver.