Although there may have been the odd exception, Jim and Jane Henson performed all the puppets on "Sam and Friends." Originally the show had no live sound or voices, the puppets simply lip synced to popular songs. Later on Jim began writing sketches and (based on all the clips I have ever seen) it appears that Jim did all the voices. This is similar to the way Jim and Frank worked through much of the 1960s.
I have never read/heard much about Jane Henson's characters but my understanding is Jim also did Wilkins, Wontkins, Mushmouth, Yorick, and most of the other "assigned" characters in the 1950s, though the actual structure of the show was probably very loose and informal.
Jim Henson's longtime friend Bob Payne took over most Jim's duties for much of 1958 when Jim Henson was travelling in Europe so it's possible he did a number of Jim's characters during that time.
I hope this helps!