It can reduce noise and improve clarity, but it cannot recover speech that was never clearly captured on the recording.
Expectation shapes perception of unclear speech, so listeners often hear what they have been told or expect to hear.
Sometimes, by examining background noise, electrical frequency patterns, device traits and metadata, especially when original media is available.
Results depend on recording quality and speech amount, and courts treat voice identification cautiously, so claims of certainty deserve scepticism.
Keep the original unaltered, store it carefully and have it professionally transferred, working only from copies.