Juvenile fantasy series where characters can play music to summon creatures to fight for them.

[R. Bryce]

This series I read as a child popped into my head. Pretty sure it was published in the 90s, but might have been late 80s or early 00s.

I'm like 90% sure that music was involved somehow, but I know for sure that the series involved people being able to summon different creatures. Powerful characters had larger and more powerful creatures.

One of the books had a cover that was a weird yellowish misty background with a figure slumped in the middle, and might also have had a phantom or ghost looking thing on it as well.

[Rainbowheart]

Who was the main character or characters?

Age, sex, etc?