Sorry, this may be a spoiler for anyone who is reading this for the first time... but I love how music is used in this series.
The many ways musicians can use their sound is something really cool, it is "bard magic" in a way, but in this setting, it doesn't use the "blue air" that makes the "true" magic.
But... the drum guy enhanced his teammate's body durability enough to take a sword slash directly, that is impressive.
Compared to "true" magic, it actually is pretty limited, but a few explanations come later.
Man... it is a true shame this got discontinued all those years back, the final showdown is pretty rushed, and I think the full potential of this world wasn't fully achieved.