I just added google voice to a sandbox and it appears to be working fine - it gets both audio and message notifications for every text received. I have the "send notifications to current user" set.
When I go back to my Admin account, I receive one audio notification from the very first Voice activity, but then no further audio, but I do receive all of the message notifications at the top. All of the notifications show up in the history too. I get no more than that one audio notification unless I go back to the sandbox and then, coming back to Admin I get that one, first, audio notification.
I did wait over a period of time to try another text in case there's some sort of timer, however >30 min and no change.
Is this the intended operation? IMO, I could be in my Admin (home) account and I'd get an audio notification for every action in the sandbox, just like the message notifications are working. Maybe I missed a config somewhere?