I have 2 TiVos, an original day one NZ TiVo (i.e. when they were first released) and a more recent NZ one (which hasn't had a problem).
1) Sometimes when I start the TV (always TV One) there is no sound and I switch to (say) 2 and then back to One.
2) Sometimes the sound has not been recorded. It may happen once every 2 weeks or so (and typically on an afternoon programme).
However, I have found that a program that is currently being recorded has no sound from the start, but when I go to Live TV there is sound. Pressing rewind and I find that the sound does cut out at some point.
If anything I would say I get a very good TV strength signal.