I'm not familiar with St.Louis but was listening to this also this morning... It sounds similar to the "hold all traffic" signal tones that I've heard on other agencies when they have a situation such as when they have a officer in distress or in this case a firefighter down.
If you hear the beeping every 10 seconds, it could be the "channel marker". PDs use them when they "hold the air" or restrict traffic due to an emergency (examples would be during foot/vehicle pursuits, officer assists, and active disturbances). Fire may use the tones in during a mayday or man down situation.
I just listened to the live feed and did not hear it. I listened to the archives and heard it. It sounds like it is the scanner itself causing the tone but I don't know why.