It’s not in the credits. (They always tell you what the special music is for that episode, but they don’t tell you what the music is that’s always there.)
Check out this YouTube from 5:00 to 5:15, and if you recognize it, give a holler - thanks!
Or for a longer example, try starting at 4:35 in this YouTube, and running for the next 30 seconds or so.
Mozart- Eine Kleine Nachtmusik
If Firebug is grooving on classical, you two might enjoy Beethoven’s Wig:
The first side has good lyrics added to the classical standards that kids enjoy and the second side plays the music plain.
OK- I was wrong- the Mozart didn’t start until 5:53 in the first clip and I don’t think it is present at all in the second. I don’t recognize the music you are asking about.
Never heard it before. Is it possible that it’s just something written for the show, and that’s why they don’t credit it every time? Is there anything saying “music was composed by/picked by so-and-so” in the credits?