Thanks for the log. The logs indicated you might be running into this issue: Coloured noises drop off in high frequencies.
Could I also get a copy of the audio file you are using to test on my end?
It may be an issue with downmixing the audio to a mono speaker as it is not noticeable while playing in stereo. Do you run into this issue with both Xamarin library and FMOD or just FMOD?
I have been testing on a Huawei and a Samsung Galaxy10 and am unable to hear a difference.