Ive experienced this on all my toons so far. Every time I complete something new from the On-Duty tab for them, the sound drops out. For example, running Area 51 for the first time on a new toon: Upon completion the sound would drop out. Same goes for the other Alerts and Raids. I'm not quite sure if it happens for Challenges and Duos, I haven't paid too much attention. Can this be looked at? It's very off putting, especially for new players who find that they have to relog every time they do something new just go get the sound back.
Happened again. On an Alt, I finished the Hive Moon Base alert last night for the first time then sound cuts out. Then today I finished Gorilla Island for the first time on that alt and the sound cut out as well.
