My bluetooth headset sounds like it is underwater?

My bluetooth headset (Sony WI-C400) sounds fine when I connect it to my iPhone, but when I connect it to my Mac, it sounds like it is underwater or something. Also, the headset has a mic in it for calls, and so I can hear through the mic if I tap it or something. Again, this usually doesn't happen on my phone. Sorry if it's a basic problem, couldn t find anything online.

The underwater sound might be caused by an improper sampling rate for the device being set. Unfortunately, I have no idea how to change that in macOS. You should be able to mute the microphone to stop the other issue.

There are two different connection formats ("Profiles", in Bluetooth terminology) for stereo headsets:
High-quality with one-way stereo audio (for listening to music) and lower quality two-way mono audio for hands-free phone calls.

It sounds like the Mac is using the hands-free profile rather than the stereo headphone profile.

You may be able to switch it in the Bluetooth properties for the headset, or if you delete the bluetooth connection and try connecting again you may be asked which connection profile to use.

If the Mac is too old and does not support Bluetooth version 3 (or 3.1?), it may just not have the capability to use the stereo headphone profile - that did not exist in earlier Bluetooth revisions.