I think there is some recommendations that you should use WAV format and export with encoding IMA ADPCM in audicity for performance. I usually merge the right and left channels down ( Tracks > Mix > Mix Stereo Down To Mono), and then set the Hz to the lowest point where I feel sound quality is still good.
I don't have hardware to test, but this was pointed out to be something that often caused issues because mp3s have to be decompressed on the fly where WAVs are already decompressed.
WAV files also don't throw this warning about being copied.