I use a Cycliq mount which has a GoPro underneath. All I did was drill out the puck that the computer mounts in and that was enough. I actually swapped it over from the Barfly mount I had previously.
I don't use structured workouts on the 520, but I thought I'd set up a simple time based workout to try it out. 5 seconds before the change of each phase there were countdown beeps. Is that what you want?
I set up a simple time-based workout on Garmin Connect and sent it to the 520. I then let it run to see what it did.
How are you setting up your workouts? Are you using the on-device interval workout?
I just bought the Varia Radar 315 and when used with my 520 plus and UT800 light in the combo mount i can't hear the beeps, when i move it to a stem mount i can hear clear as day. Frustrating that I have to use a work around like this given the investment in a garmin connected bike.