Any tips for Cubase 12 Ai for Compressing to reduce hiss?

Hi there, I want to import pre-recorded files into the new Cubase AI 12 - however even though the mp3 files sound fine WHen played outside of Cubase) when In there is horrendous hiss and high frequency 'noise… Have been trying to use the Channnel strips facilities (from the Mixconsole) but my inexperience with it is not improving things so far. Any tips or advice would be greatly appreciated. Thanks for reading …