Seems to me you'd be better off with a circuit that up-converts 20 to 100 Hz sounds by a factor of 20, putting them into a range you can hear. That would not have to be a complicated circuit, and no doubt they're already out there.

Or it could convert sounds in that range into something that amplifies those frequencies and vibrates against your skin instead (a 'skin speaker') ... probably easier to make than a converter.

Not suggesting that what I'm about to describe is a common reaction but I'd be a little uncomfortable with the privacy implications of the setup. Is there a light to show it's on? How about a physical switch to completely interrupt the mic's connection when it should be off? Don't need a bored nerd down the hall with a Flipper tuning in for some free entertainment in the form of my colonoscopy results.

Rtings.com provides frequency response charts when they test speakers, let me see if I can find one that goes low enough for you.

As for the bonus question, absolutely love it. I love when people come up with cool solutions to life's problems, and this lets me hear my own heartbeat? Hell yeah!

Here's an easy way to check a bunch of them quicky, the Denon home was the best I found in terms of the very low end. I'm sure someone else can do better, I'm not that much of an audiophile and know very little about speakers.

Probably worth noting that they stretch out the low end of the chart so they tend to go lower than it seems (I assume it's a logarithmic scale). You might be able to go to a store and ask to test them yourself.

Second edit: if headphones are at all a possibility (they might be better for patients who don't want to hear their own heartbeat) then can I recommend Skullcandy crushers? They're completely ridiculous as regular headphones, they basically just have a metal plate they vibrate for the bass, but it goes all the way down to 20Hz, and you can crank it waaaaay up with a slider on the side. (I use these daily for music because I'm a bad audiophile who wants to have fun sometimes, I have a proper wired setup if I want the audiophile experience.)

I'm not a medical professional nor do I use hearing aids, so I might be misinterpreting the situation here, but are the speakers actually the problem? Presumably the other nurse's headphones were capable of playing the sounds in that range; the problem was your hearing aid. How does having another speaker playing those same sounds help, if your hearing aid still isn't able to correctly handle them?

It seems like what you'd actually need is something that can take in the sounds in the 20-100 hz range, but play them back in the 100-3000 hz range, so your hearing aid can pick them up, no?

I dont know if they fit with your hearing aids, but maybe look into bone conducting headphones. I have Openrun Shokz Pro and they work pretty good with music. They have a waterproof version which is good for your hygiene needs.

Your Bluetooth speaker around the neck idea might be hard to achieve. The wavelength of a 100hz signal is around 3,5 meters and is getting bigger the lower you get. This means you need more energy and bigger drivers for true reproduction of low frequencies. A 100hz speaker might get heavy around the neck.

Sidenote: When I'm wearing my Shokz I sometimes get asked if I wear hearing aids.

I haven't any idea how they'd work with your hearing, but what about bone conduction headphones? They can be wiped down easily and are often waterproof.

Tried bone conduction headphones yet?

https://www.techradar.com/news/best-bone-conduction-headphones

I got the Sony SRS-XB10 specifically because it has a range as low as 20hz.. not sure if there are newer ones out that are better now

Double check that the problem isn't actually your hearing aid's Bluetooth codec support, rather than the physical driver's supported frequency range.

Bluetooth audio devices need to agree on an audio compression format before they can send audio, and the default lowest common denominator codec that all audio devices support is extremely bad at low frequencies (and is all around terrible). It could be that your hearing aid is good enough but doesn't support whatever high-fidelity codec your coworker's stethoscope supported. Investigate what codecs your hearing aid supports and what the stethoscope supports. You'll probably want something like LDAC, LC3 (aka LE audio), aptx HD, aptx adaptive, or preferably even aptx lossless, to be supported on both devices.

If you're considering investing a lot of money anyways, you may want to consider replacing your hearing aids with ones that support this functionality (the frequency response and the codecs).

As someone else already pointed out, if your current hearing aids don't support below 100hz, then even if you play the sound over a speaker, your existing hearing aids are just going to act as earplugs. You may need to invest in different hearing aids or just live with taking out your hearing aids for the foreseeable future. Or I suppose just blasting the heartbeat over the speaker to bypass your hearing aid (caution wrt audio feedback lol)