Either the speaker itself, or too little or too much voltage to it. I test audio tweeters with a multimeter set for ohms. If they read 1 to 10, still ok.

Could it be because of low audio frequencies? Perhaps a combination of an old speaker/damaged? and low frequencies?

Have you tried to enable the wqualiser high-pass filter and test some frequencies?

What decoder project did you use? Or did you use the generic T44 sound in the sound library?

If you take a look at the 37946 project, you will see that Marklin has tweaked the project and is using the equaliser. But note that your speakers might need a higher frequency in the high-pass filter setting. might be that the new project is also tuned to reduce resonance?.

Also worth noting is that there have also been a few occasions where Marklin has wired two speaker locomotives incorrectly.

Personally, I have only compared the 37946 with the 37942 and some Di 3 nohabs, but it was a nice improvement. I did upgrade the 37946 with more sounds, but I left the EQ settings untouched etc.
And it sounds like märklin has used the same engine sound on the T44 and the Di 3 sound projects.......
Note that the older t44 and di3 engine sounds contain far more deeper frequencies. E.g. I tested the big ESU 50344 speaker in one of my Di 3's, fitted with mSD(1). However, the speaker struggled with the low frequencies, and I got a lot of distortion, so I had to reduce the volume. So not recommended on a mSD, but I plan to fit it with a mSD3 and tune the equaliser, so I can take full benefit of the speaker.

But primarily, I have steam locomotives, and recently I upgraded the decoder in my 37051. mainly to improve the motor control of the maxon motor.
I also took the opportunity to improve the sound. I replaced the speaker with a better conventional one and used the EQ to tune a BR 05 project to the new speaker. (the new speaker fitted in the old position, but was thicker)
So I gained some decent improvement in the lower frequencies, but if I used too much gain etc, I got sound distortions, as small speakers have limits.

Unfortunately, I'm no sound expert, but with a CS3 and a roller test stand, it makes it fairly easy to make basic adjustments to the EQ as you can hear the changes immediately.