Sorry for not responding, It's been a bit like a circus over here, no balloons unfortunately.
To be honest, I don't know much about buzzers. I would think one could attach a buzzer the way you like, but I'd check how much milli-Amperes will flow by hooking it directly to 5V.
More as 40mA on a pin might damage the controller. In that case you'll need a resistor (making the buzzer less loud) or transistor capable of driving higher currents.
100.000 feet sounds great, living close to the beach I met some guys taking KAP-pictures (Kite aerial Photography) last year. They didn't need a buzzer, but... didn't get much higher as 1200-1400 feet. (still very impressive and great pictures)
Thanks for explaining how you do track the balloon, it's interesting to read. A few "centuries" ago I've been sysop of a packet-radio-BBS on CB for a few years, every two weeks we had an ARDF-contest with Hams, so my first guesses were Yagi, Parabolic antenna or GPS. But... Internet became available, nr. of users dropped rapidly and I took a left turn where I could have turned right.
73+55 :),
Bart.