In archaic words, an mh often represents a transitional sound between unmutated m and fully mutated v, and is pronounced as a nasalized [v]. In examples like the above, however, it is simply an alternate way of representing a mutated m, and hence is pronounced as an ordinary [v].

Fiona Jalling’s book is the most recent Sindarin textbook and is quite good. You can get it here:

https://realelvish.net/store/

+Paul Strack Thanks for clearing up my question. I meant Quenya textbooks but that Sindarin book seems interesting!