Oh, man, I hate to be the pain in the butt…
But it looks like you have some severe lossy compression in the lineage here. It shouldn’t look like this from good analog recording. Or you are somehow recording the Dolby Surround track instead Analog mono, idk…
It should go all the way up to 22kHz.
it is quite possible that the spectrogram is 100 percent accurate. Some audio sources don’t go above 16khz (it all depends on the source material)