Do Beholder Beasts Have ears?

[Waker]

What if the bard can see the beholder through a mirror, the beholder can see him through the mirror, but the beholders eye isn't pointed directly at the bard? Wouldn't the music eventually reach the beholder's ears?

And also, wouldn't you say the bards ability is bolstered if he is able to naturally "amplify" the loudness of his music?

I would allow the mirror trick I work were I the DM, provided that the Bard was still within 90ft of the Beholder. RAW-wise though, I have no idea on the ruling of that.
Loudness isn't really a factor for the effectiveness of the song. At most I would say the volume would only counteract any other noises that might be applying a circumstance penalty to the Performance check.