In-monitor speakers are usually tinny. Instead of relying on them, have your AV specialist connect the video recorder directly to an amplifier and two or more speakers. Even simple amplifiers and cheap speakers will make an enormous difference to audibility, as well as giving you control over channel selection and tone. Routing Supplementary commentary may be essential to some students, but distracting for others. To deal with this you could arrange the speaker for the channel carrying audio subtitling to be located near the VI students workplace, and to have a jack socket for a headphone.
Headphones are a useful option when private viewing is taking place.