I'm not aware of a specific word that would be well-understood to mean what you want. I think what you would use to get the idea across would have to be something approximate, being either slightly off or slightly too broad.
I personally would choose "amplify", given the context you suggest:
- I am using this stethoscope to amplify your heartbeat.
This isn't quite what you're asking for, but a typical english language speaker would definitely infer "listen intently, at an enhanced volume" from "amplify".
Other options just seem too far from what you're asking. The most direct verb is just "hear", and clearly that isn't what you're looking for. The closest thing to "visualize" would be "hear in my head" and that's not even remotely what you want.
There may be some esoteric word that does mean exactly what you want, but I suspect it's not well-known and thus may not really be the best choice, unless this is meant for a very specific audience that is aware of its meaning.