Yes: your ears... Seriously.. 'denoising' is as vague and blurry as
'enhancing'. As people have already suggested there are many different
approaches and consequently tools. Experiment a lot and see what gives
the best result for what you need.
'Cleaning' aim is, in my humble opinion, #1 to consider: e.g. do you
want to make the voice simply as intelligible as possible (in this case
you don't care about possibly destroying its timbre with funky EQ) or do
you want to maintain its character as much as possible (in this case you
have very low room for extreem EQ..)?
> on linux?
Yes. There are many powerful tools, but first try and decide which
routes you'd like to try (e.g. if you need a noise-gate, an eq, a
(fft-based) denoiser, etc.)