Sounds like digital compression artifacts. It's probably happening in the music as well, but it's a bit less noticeable there because there is more going on as things decay. You could verify this by listening to the analog and digital versions of a local station.
Are these real radio stations, or someone "broadcasting" out of their house? I'm asking because an on air studio will be quite acoustically dead, but an internet station might have less effort put into such things. Even the 300watt giant I worked for had a near silent broadcast studio.