I’ve seen this a few times and it is indeed a poor seal on the rear speakers. On those occasions that I’ve heard of it, it’s all been within warranty period and the dealers have sorted under warranty, so it’s certainly something VW are aware of.
To be honest, I’d probably sort this myself (as I don’t trust dealer workshops), by drilling out, the rivets holding the speakers on, replacing the rubber seal (or use mastic or similar to seal around the speaker) the re-rivet the speaker back on. Sounds like an easy enough job ?