Sound Waves are not able to be soloed in a new project or unsoloed in an existing one from a prior version.
This was reported and tested in 4.24.3 (CL-11590370). This was reproduced in 4.25 (CL-13144385) and Main 4.26 (CL-13544465)
Results: You can't set the sound wave to Solo. If upgraded from a prior version you can't unsolo a soloed sound wave
Expected: To be able to solo/unsolo a sound wave
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-93554 in the post.