[Rendar]

Sound Wave, a Bard skill, has the "Note" tag, like Dark bard songs do. However it does not take durability to use (implying it's an instrument skill), and also does not benefit from Volume Up's range increase on song skills.

Using Sound Wave, also, breaks the Singing status.

[Neus]

Not a bug. The icon you refer to just means it requires an instrument to use. It's not a song skill, so it breaks Singing.