I can no longer hear certain audio on some of my projects

So for a little context: About 6 months ago I was working on a few projects that involved video, voice, music, sound effects, etc. I went on a hiatus from them and I'm just now revisiting them, except there's a problem. Certain blocks of audio do not play any sound at all. All of the voice audio is fine, most of the sound effects are fine, but this problem really only applies to the music. The audio on the timeline shows up completely normal. The sound levels are just how I left them, the waveforms are even still visible, but whenever I play that portion of the video there is no music, just the detached audio from the video and the sound effects. They aren't red either, which means that it's not a problem of the sound being misplaced. I double checked that they were in their original file location, and they are, and when playing the audio from there, it sounds just fine. Even when creating a copy of the original sound and replacing it in the file location, the problem persists (when going to import on fcp, the previewer for the sound doesn't even work). Originally I thought It was a problem with them being mp3 files, but then I realized that the other stuff like sound effects are also mp3 files and they work just fine. Something I found very strange though was that there are a couple pieces of music that still produce sound, but they are no different than any of the other audio. They are mp3, in the exact same file locations, even on the same lines that other pieces of music that aren't working are. I really can't pinpoint exactly what happened to the audio and how I could possibly fix it, or if some strange accident occurred where most of the music, and only the music, across multiple projects stopped working. Any help would be much appreciated!

Did you ever resolve your issue? I know that MP3 files can vary drastically in terms of settings used to make the files, so some MP3s may be okay to use, whereas others may cause issues. If you were to convert the source MP3 files that were causing issues into WAV files, would the issue go away? Others have reported quirky behavior with certain MP3 files. In some cases, when media is imported into an FCP Library file, rather than being left external (FCP's "leave in place"), issues can arise...

I've been having this issue a bit, too. Haven't found a solution. Have resorted to editing the same clips back into the timeline. This isn't just an MP3 issue as I've had it happen with video clips once I've detached the audio for advanced edits.