The Music Box is not that big of a theatre so sitting in the balcony should be fine. There is no separate Mezzanine according to my seating chart, just one balcony with 11 rows. Balcony row A overhangs Orchestra row J.
Gypsy is right, its a great small little house-perfect for a 2 hander, so dont be too concerned where you sit. I saw the flop musical Swinging On A Star here awhile back and sat 5 rows from front of the Mezz-perfect!