Passenger door speaker problem

[Rondo]

During this spell of hot weather I've noticed that my 2011 Fiesta passenger door speaker makes am awful distorted crackle particularly noticeable on low notes / bass. At first I thought the speaker had blown, but its fine in the morning before the heat has set in.

In searching around for explanations, I discovered I am not alone with this problem, but alas no viable solutions have been put forward so I thought I would ask here if anyone else has experienced this and been able to resolve the issue