So, basically how music discs work now is you have to set the amount of time they play for in ticks. if you don't do that, it will only play for that amount of ticks before suddenly stopping.

This 100 is the amount of ticks aka duration. Recommend using the minecraft wiki calculator (https://minecraft.wiki/w/Calculators/Tick) to convert the song's playtime into ticks. Also add one more tick to account for leftover milliseconds.