Musicianship tooltip says
"Gain EXP based on the number of notes played during a performance."

However it only gives you exp based on how many measures (length) used.

Steps to reproduce / test :

1- Check leaderboards to see your current music exp

2- Play a song with 2.4k Notes - 100 Measures

3- Check leaderboards to see how much exp you got (0.02% increase)

4- Play a song with 300 Notes - 100 Measures

5- Check leaderboards to see how much exp you got (0.02% increase again)

6- Play a song with 2.4k Notes - 200 Measures

7 - Check leaderboards to see how much exp you got (0.04% increase)

200 BPM and 3/4 Meter used for testing but its just to speed up the process so it shouldn't effect the outcome.

Sadly leaderboards seem to be only way to check your exp due to exp gain being incredibly small, as your profile doesn't show decimals.