Don't know without being able to see your forum. Please fill out the SRT.
How can it be a bug on Windows when you have the problem on an Android phone? If this was a Widows bug with phpBB boards then do you not think that there would be thousands of people complaining about it?
Honestly I faced the same issue time ago, so I am quite sure of what I am talking about.
Define sound effect.
It happened on his phone as well though.
In fact my smarthpone was near my PC which has the speakers on but the cable of the same speakers was unshielded, so interference (my smartphone was in use in Tethering, so for those who know this is an important detail). Here we talk about other things but not phpBB.
Betamax if you please, nothing otherwise.