When adjusting the mike gain with the top cover of the rig removed, I found the sound of the built-in speaker much more pleasant to my ears. The "hollow" sound of the empty case and the resonance of the case to some audio frequencies completely disappeared.
- Find a plastic bottle with 35 mm diameter and cut the top off. Plug the bottom part in the hole in the panel board behind the speaker (I used a 60 ml contact lens care-product bottle, which fits snugly).
- Fill the space behind the speaker with a piece of mineral wool, wrapped in a plastic bag for dust protection. Do not fill the space over the main board between the front panel and the heat sink!
It's almost like using a DSP-Loudspeaker!