I could hear all frequencies, with my Sennheiser CX300s, the TV on the other hand topped out at 16khz
For the first 3 months of riding, I didn't and each time after riding a fair way, my ears were ringing a little, I have recently got some Sennheiser CX300s and they are the ones that fit securly in ear and they are great, reduce the noise so much with out music, then I can put music on at a really low level on my MP3 player and hear it perfectly (and I can still hear everything around me) the best of both worlds.
Only time I don't ride with them is if I am just going to be riding 5 mins down the road in low speed areas.