I have listed several euphonium players who were featured in movie soundtracks in the Euphonium Music Guide.

But there are sometimes euphoniums in the soundtrack that, while present, are not featured. Here are a few:

Flags of Our Fathers (2006)
Steve Holtman (trombone and euphonium)
Bill Reichenbach (bass trombone and euphonium)

Tora! Tora! Tora! (1970)
Dan Cerilli

Jurassic Park (1993)
Dick Nash (trombone and euphonium)
Bill Reichenbach (bass trombone and euphonium)

This site has lists of the brass players in movie soundtracks (which is where I got the three above):
http://brasssectioninmovies.blogspot.nl/