I’ve used Audacity in the past with no problems but recently after recording some music the sound if wavy and muffled during playback. I am recording from a cassette deck into my computer using a patch cord with a 1/4 stereo plug from the cassette deck and a stereo mini plug on the other end into my computer.
Make sure Windows “enhancements” are turned OFF. They can create strange effects when trying to help.
Are you connected to a line-input (blue on a regular soundcard).
The microphone input on a laptop won’t work properly. Line-level and headphone-level signals are about 100 times stronger than microphone signals. And the mic input is usually lower-quality and mono.
If you don’t have line-in you’ll need a USB audio interface with line in.