Premiere Pro audio muffled and distorted on export

Might be because you are exporting a stereo audio file which has one channel phase reversed, and then it is being played back in mono.

If you have an identical signal going to the left and right channels, but one of them is phase reversed, it means the waveform is inverted (upside down). When you listen to either channel on its own, it will sound normal. When you listen to them both in stereo (Left channel to left speaker, right channel to right speaker) you may notice it sounding odd. Some people describe it sounding like the location of the sound is "inside their head".

But when you play back a stereo recording like this in mono (left channel and right channel both mixed togther to a single mono output) the signal will completely cancel itself out.

If the two channels are almost identical, but have tiny differences, only the differences will be audible. This often happens with lossy file formats (like mp3/mp4 files), where the mono version of the phase reversed playback sounds tinny and lacking in all bass.

To test it out, try adding a channel mixer effect to the master output of you mix, and inverting one channel. Then export again and see if it fixes it.