I once saw a chap use a solid rod of wood pressed against the hive body and the bone just behind his ear. He said it transmitted the sound as if he had his ear pressed against the hive. I tried it but couldn't hear anything, but I didn't persist.
I find a stethoscope rather uncomfortable in my ears, but ou can also use a listeningc tube, i.e. any small diameter flexible pipe - just put one end in the entrance of the hive, or between frames if it's open, and the other end in your ear. The tube should be small enough that a bee cannot crawl in 8)
And it's just occurred to me you could probably use a funnel as a crude ear trumpet - put the large end against the hive wall, it may concentrate the sound? Could stick a flexible tube on the other end for more comfort / more ergonomic.
A totally different approach would be to run a spectrum analyser program on your mobile phone - for example Spectroid is a free app - and place the phone so the microphone is in the entrance, you would then SEE the change in tones.