I can easily hear (and see) mine with the radio on at volume 14, which is quite loud enough for me. If I have passengers then I need to turn the radio down from that setting.
That's higher than I have my volume set but it depends on the source.
In any case I just went out into the garage and used a sound meter app on my phone. Now obviously the absolute readings can't be trusted as we don't know how well calibrated it is. But we can reasonably rely on the relative values it reports so here goes:
Sitting in the car in the garage with nothing switched on: ~40db.
With me talking in a normal voice: ~75db.
With the radio and engine on (actually playing from my iPod): ~65db.
With just the parking sensors (easy to trigger from inside a garage) and engine going : ~50db.
With just the engine running: ~50db.
So the proximity alarms are so quiet that the sound meter doesn't register an obvious difference when they are on. It also doesn't help that one of the alarms is quite high pitched.
If you want to repeat the experiment and have an Android phone here is the link:
https://play.google.com/store/apps/details?id=kr.sira.sound&hl=en_GBIncidentally it registers ~30db in my lounge when I'm quiet and ~45db when I'm typing on my laptop. So the proximity alarms are not much louder than my typing. That's too quiet. Now personally I can live without proximity alarms as about the only time they go off is when I park in the garage. But since they were included in the price of the vehicle I don't see why it's unreasonable to expect to be able to hear them over other sounds.