If you have the new version of Vymail and you listen to a voicemail through the handset sometimes it changes the sounds from other apps to the handset.
Try going back to Vymail and listening to a voicemail and change it to the speaker output.
Had this happen to me with the stormsaber app where it was playing the sound for it through the handset. Other than that I suppose it could be due to the known bug in .75 but i doubt it.