Playback volume for voice parts almost inaudible

Hi all
I've just noticed on a bunch of my scores (but not all) that since upgrading to MS4.5, voice parts are almost inaudible, although mixer settings are quite normal. Changing the part properties to any other (non-voice) instrument results in normal playback volume.
I've attached a small piece from one of the compositions: in this piece, the bass is also inaudible - the only audible parts.
Is this a bug, or something wrong in my setup, or some weirdness in the file itself?
Thanks in advance