I repaired the OS/B file I have, the silent part of music was caused by a missing sector 128 zeros which should've held data, and there were 2 more bad bits. The file size 14912 should have $0A at offsets $5F2 and $619, instead of $09.
I compared 3 versions: OS/A file here, OS/B on a disk, and broken OS/B on a file size 14912. All 3 are supposed to differ in 2 regions only, at small code routines found both at the beginning and at the end. The 2 byte sequence "09 d0" appears 4x and the sequence "0a d0" appears 3x. Bad bits caused 09 d0 to show 6x and 0a d0 to show once.