Hello!
I have some poor audio quality recordings at home (live recordings, cassette etc) and I'd like to enhance their quality a bit.
Do you have some intresting techniques to do that, any favorite programs (I run Linux), may be you know a nice website? Any help would be much appreciated.

I guess you still need to digitize the recordings. Indeed, once digitized, the recording can be preserved without further quality loss. It can even be "improved" somewhat, but do not expect miracles: a poor recording remains a poor recording.

You probably will need to learn a good audio editor. Audacity is a nice choice, running on a variety of platforms, not too difficult to master and nevertheless quite powerful. For Linux, there is also Ardour, which is as far as I know of a quite professional level, but also quite a bit more difficult to master. I do not have experience with it.

What you need to achieve in the first place is a digital recording of your recordings played back in optimal circumstances. Head of tape recorders must be cleaned, pick-up stylus must be in mint shape, the recording equipment should be the best you can.

During the recording, you should keep the signal/noise ratio as high as you can: this means that you should maximize the recording volume, while avoiding any clipping. However, play it safe: it is better to have the recording level somewhat lower, leavng some headroom, than risk to introduce clipping in certain spots.

You preferably record in 24 bit or 32 bit float if your computer is fast enough (should not be an issue nowadays). Then you can downsample at 16 bit at the very end, after all processing.

Anyway, you best have some readings on the net and get started: it is the best way to learn.