Try http://www.espn.com-horse racing, you should be able to watch some US races. I used to use http://www.northamericanracing.com but it seems to have disappeared.

CP
Do you listen to UK races on willhill radio?

IMO you can't beat UK races for value, volume & volatility. A word of warning re US races, they don't always start at the advertised time. I've seen them jump 30 minutes late, but also go off 10 minutes early.