My audio file sounds crusty and muffled

So I have two audios; an mp3 text-to-speech track, which sounds perfectly fine, and a song downloaded in mp3 which sounds crusty and muffled when I play the track. (the song track is two separate tracks if that helps).
Normally the song sounds fine and I have the same version from the same website on my phone and it sounds ok but, when I put it into audacity the audio quality decreases by a lot. It also sounds lower and ‘grainier’.
I’ve looked through other questions but nothing really seems to fit my problem, and I have tried the things they suggested on there but it still didn’t work properly.
I don’t think the problem is the mp3 because the tts sounds fine and isn’t changed at all.

Your MP3 may go over 0dB (the “digital maximum”). There was an issue/discussion here a couple of days ago where somebody was trying to “overdrive” and clip/distort files and Audacity was behaving “differently” than other software.

Is your Audacity configured to [u]Show Clipping[/u], and do you “see red”?

If so, make a copy and open the copy in [u]MP3Gain[/u].
Click Modify Gain and then Apply Track Gain. That should bring the level down to a “safe” level. (MP3Gain directly alters files so it’s best to work on a copy.)

Just checking to see if the floor is level… Are you listening to the phone and to the PC via the same headphones/earbuds?