Being a new user of this site and of Phonak HA’s I have not seen a reference to this topic, although a search brings up some old threads. This might be of use to other newbies here.
I recently purchased a used Select In from ebay, and it intermittently went silent at low volume.
I came across this post today and did an upgrade to the firmware and it seems to be working better.
https://www.phonakpro.com/us/en/support/product-support/wireless-accessories/roger-upgrader.html