To answer your questions, no, not all dogs with SM yelp. Murphy has CM, with a pre-syrinx, and has never yelped, Misty has a large syrinx at her neck and has yelped in the past before we got her on the right meds. It really depends how severely the dog is affected, and how it responds to the pain. Both of mine make contact with the skin when they scratch, they both tend to scratch their ears, and Misty scratches her elbow on the side her syrinx bulges out. Misty only bunny hops when she's on the lead and trying to keep pace with Murphy and scratch at the same time.
The thing is to try and not worry, and not to assume it is SM, I know it's easier said than done, most of us have been there. I'd finish the treatment for the yeast infection and see how she is. If she's still scratching, then go back to the vet and get them to check for mites, and to make sure her ears are better. Does she have any red, sore looking skin under her chin? Is she up to date with her flea treatment?
When did the scratching start? Has anything in the house changed? Has her food changed, or treats?
There are lots of things that can cause a dog to scratch unfortunately, so try and rule them out before going down the SM route
Paula - mum to Murphy(6) & Misty(7), and Jerry our cat.