Interestingly enough the same thing happened to me last night, I lost
sound on my Sky (SCART) channels and sound+picture on all of my RF
channels. Cables were all connected well, and I'd had no reboots inbetween.
I went out, came back and found that X-Men 2 had recorded without sound.
Viewing my Sky box directly I got sound, but when watching Live TV via the
TiVo and playing back the XM2 recording were both silent. Also I lost my RF
channels, TiVo couldn't pick up a signal from them at all.
In my case a full reboot sorted it.