A quiet little village, Muang Kham is pleasant enough, but interesting really only for its proximity to two minor local sites.
These can be just as easily visited on day trips out of Phonsavan, which is what most nearly everyone does, so if you're in Muang Kham, Phonsavan is obviously way too mainstream for you. Also, if you're heading in the direction of Sam Neua and want to see Tham Piew, Muang Kham can save you a bit of time on your journey.
The sites in question are a cave and a hot spring -- neither are earth-shattering, but they're more than adequate diversions.
First is Tham Piew, a large cave on the road to Sam Neua that 400 civilians used for shelter during the Secret War. One day in 1968 US aircraft lobbed a rocket in killing over 400 civilians who were hiding inside. There are memorials and a museum onsite and charring from the explosions (four rocket in total were dropped, three missed) can be seen on the exterior of the cave.