Could an untreated abscess on a tooth lead to mild hearing loss and a ringing in the ear?

No: Anatomically they are far apart. Unless the infection from the tooth somehow seeds in the middle ear, it should not normally cause hearing loss.

It shouldn't: But it can lead to a host of other problems. Get it taken care of asap.

Kind of: An infected tooth, particularly a lower back molar will cause nerve pain from the jaw that radiates to the ear. So it will feel like your ear hurts which can manifest as a clogged feeling or ringing in the ears. Actual spreading of infection and pus to the ear is unlikely and would certainly appear very alarming.