Well, comments have been made in other posts, that the sound system in the Murena at cruising speed is provided by the engine, as no stereo can overpower that.
Thinking about that I decided that it was just a matter of the insufficient power handling of the original speakers. They were cheap, low power paper cone, with a pewny magnet. I took them out and measurted the mounting depth available and ordered a pair of Monacor CRB-130CP coaxial 80W speakers. They should have fit, but didn't, because the more powerful magnet was too large in diameter. In the chassis there is a cone shaped indention just besides the foot rest (if your car has such a thing) and on the passenger side too. It looked like there was a plastic ring glued into this indention. First I thought it was preventing the speaker from fitting. I successfully remover it, only to find out that it was a plug that had been inserted from the OUTSIDE and I destroyed it in the process.
That is why there is now a new, home made, nylon plug seen in the photo below.
After surveying the options it became clear that the only way to mount these speakers was to put a spacer underneath them. But, alas, the foot rest did not provide enough clearance. Move the foot rest? No. The foot rest is welded to the chassis from what I can see. Solution. Cut 2 cm off the side of the foot rest with a power saw. First I cut off the side that was bent up at a right angle. Then I cut 2 cm notches in from the side at top and bottom of the flat part. Then I bent the 2 cm flap up to 90 degree instead of the piece I had cut off. Finally I cut triangular pieces off the top and bottom part to meet up with the new bent part. The result is that there now is an extra 2 cm of space for the speaker with a spacer behind it. It all fits with 4 mm to spare.
The first photo shows the new nylon plug
and the foot rest after bending up the new lateral support up and cutting the top triangular piece off. On the bottom of photo 2, you can see the original width of the foot rest. At the top of that photo you can see that a triangular piece has been cut off flush with the new bend. the bottom two photos show the finished result.
This new setup (there is also a set of speakers in the doors, mounted by a previous owner) plays plenty loud and clear, to provide pleasant sound above the engine, even at 120 kph.