Radio stations that have internet streams I listen too usually don't play ads, at least not on the stream. I know this is partially for legal reasons, and partially because it's it's a pain in the ass even for large radio stations to have specialized ads for their internet stream -- not to mention small time internet radio stations and podcasters.
I don't know why the magic of contextual advertising hasn't been applied to online radio yet, but it would probably be easier than it sounds. Though maybe harder for podcasts than for streaming radio.
Companies like Google that are all ready good at data mining could feed an audio stream through an AI that would determine a context of the audio and send specific ads to listeners during commercial breaks. Though that wouldn't be necessary. You could probably do just about as good with the name of the show, songs, and standard location information about the listener (location of IP address), etc. It picks from databases of audio ads, plays them for the user.
The difficult issue would be bandwidth and playing specific ads for users. Server software would need to be changed to communicate with the ad-embedding companies servers to get the information and then send specific ads to specific listeners. Failing that, companies interested in contextual advertising with audio would need to setup their own streaming servers for stations to use. Either of these options are in the grasp of Google-sized companies, and would produce a fairly high return on their bandwidth. AOL could easily do it with WINAMP/Shoutcast.
Podcasts would be more difficult since they're pre-recorded. But, the shows and their metadata could be ran through the AI after being produced and ads recommend for commercial breaks. Podcasters would have to create some kind of contract with the advertising company. Or the advertising company could host the podcasts and embed their ads within the podcast to guarantee to advertisers that they're being inserted and listed too. Of course, this also re-creates the TiVo problem -- people could fast forward through them. That doesn't seem to have deterred television advertisers, however.