Well, without hearing your recordings it's hard to be specific but I'd probably start with some equalisation. If it's your first try, the 30 band Graphic Equaliser is likely the easiest to start with.
You'll likely want to cut most of the frequencies below about 200 Hz, maybe boost the mids and upper mids just a bit, and (as you start with a cassette) you can probably knock off most of the frequencies above 10 or 12 kHz since they'll probably be mainly hiss.
For the exact frequencies you'll have to use your ears to judge what you need more or less of.
If you like, you might want to try posting a quick sample of the problem track (on a site we can download from, not just listen to) and we might be able to come of with some other ideas or at least be more specific
One caution is not to expect miracles. The real world is more limited in what it can do than Abby on NCIS!
