This seems to be a common issue when combining the Varia RTL515 and the mobile app - volume. I've ridden with the Varia probably 20 times now. I have an iPhone. It's always been very lightly audible, hardly helpful though. Only one of those times was it perfectly audible. I had changed nothing. It signaled for an approaching car, then gave a lighter signal after the car passed (and no other cars were following). I thought, "Oh, this is how it is supposed to work!". Next time I went out (again, nothing had changed), it was completely silent. I have noticed that the ringer volume makes no difference. However, there is a volume control image that comes up on the left side of the phone display when I play something like a podcast. I'm wondering if this has something to do with the proper function of the app. Without sound, I feel it's hardly worth it!