The noise is abrupt enough that Frisk nearly laughs. It might not be funny, though, so they clamp down on it hard, and they keep looking for the voice. "Welcome!"

...The voice they might kind of know. Something about it's incredibly familiar. So either a Chara or a Frisk.

Time for some slightly shout-y (on their end, at least) conversation. "Did you get a music room guitar?"