I don't know if this issue affects all Android 2.3 devices. If you set Silent mode ON, the sound of your app is not muted. It looks that these devices do not mute "media" type sounds like music from mp3 player (it's correct) but Corona probably use the same type of sounds.
In my opinion, it's not so big problem but it's a reason for rejecting on some app stores - and this can be a big problem :)
Maybe, there is a way how to detect Silent mode ON state, but I don't have an idea how to do it...