The audio stream and video stream are combined (muxed) at the source and broadcast. The TV tuner splits them apart (demuxed). When a data packet is received but corrupted both the audio and video are effected. As you know with digital data you can easily corrupt the data by changing a few bits. So you lose not only the few bits but the entire package of information. Plus with MPEG video it requires a few packets to build an entire image. With analog there are no packets. Analog signals are a different beast. Basically your TV can lock onto the analog audio signal easier than the analog video signal because there is less information encoded in the signal to extract.