P9a on latest, can confirm if you set a system default sound for notifications and a specific one for SMS. The default sound will be played. However, if whatever SMS conversion you want with the diffent sound shows up in the Message app notification settings and you toggle some other notification and back again to what you really want.
I opened a bug report on this. Also, the P9a is currently not in the main dev branch. Notice other bug that were fixes on my P6a are a issue again.
For me Signal works as expected with a specific sound.