Missing parts in wav.file

Ok so Iv'e been working on a piece for about a week now. I had a rough draft and made a wav. of it and all the parts were there playing, Trumpet horn trombone baritone and tuba. but now that Iv'e completed it and made a new wav. of it and now the horn part does not play. It plays in musescore but it doesn't play in the wav. anymore. Maybe I muted it somehow by accident?