Sounds like this is another case where seller will not hold his words for 'personal' warranty, and a hidden agenda?

You do not have any proof that the speaker is alright before you passed it to buyer. He did mentioned that the cosmetic look of the speaker has been in terrible condition too.

If this thing brought into police station, I guess it will be hard for victim to have an advantage.

Firstly, police does not know anything about sounds system, for them, janji ada bunyi can already.

Secondly, seller can frame buyer for putting up his personal information and bla bla.

I guess this first reason above is enough to put seller on higher ground here.

Good luck TS, I really do hope that this can be solve by having middleman here, but I guess seller would not do that.