Need a bit more info ....
* what's your video source (cable, cable box, satellite receiver)?
* how is that source connected to your Tivo?
* how is the Tivo connected to your TV?
* when you're able to clear the problem, do those "bad" recordings then have audio?
Sony SAT-T60 120GB (v3.5b)
HR20-700, HR20-100, HR21-200 HD DVR
HR10-250 HD DVR 400GB (v6.4a)
H21-200 HD Receiver
R10 DVR 160GB (v6.4a)