There was a 30-minute documentary called Lava Lake Acid Lake by volcanologists Maurice and Katia Krafft. The Kraffts were famous for close-up photography of lava flows, and in fact, died in Japan in 1991 (along with 41 other people) when a lava flow suddenly changed direction. Their documentary appears to have been released on disc in 2006 by Learning Media of America (Las Vegas) - that company, which distributed educational films for free, may now be out of business, their former web address (lmofa.com) having been subsumed by someone else. The World Catalog shows a few libraries with the disc, so you could ask your local library to try to get it through interlibrary loan. At most public libraries this is a free service, but sometimes holding libraries won't lend something if they consider it rare.
The BBC's 2008 Earth: the Biography covers volcanoes on the first of 2 discs, and the lava lake in Ethiopia is covered. It is available through Amazon, and many libraries have this one, so you should have no problem getting it.
The Nova program (PBS) produced an episode called Volcano Under the City:
Will Mount Nyiragongo Obliterate This Thriving City? Nyiragongo is famous for its lava lake, so it is probably discussed. The same volcano is covered in the 2011 National Geographic Explorer series, Season 1 Episode 13, Man vs Volcano.