It's not clear from the recording, but I'm betting these were the small party-type balloons one would buy at the store. Maybe somebody had a grand opening, or a big birthday party, etc. and they released a bunch of balloons? If they were a bunch of full-size hot-air balloons, this would have been a much bigger deal - and ATC would probably have had them on radar!
Hmmmmm... in the US, balloons have the right-of-way over powered aircraft. I wonder if that applies to toy balloons, too! ha ha