Discrepancy in what people are hearing in output MP3

[Tsjanith]

Sorry if the title is a bit vague, but I really don't know how else to describe it.

So I have a client who I am editing podcasts for. I did the editing, normalized the LUFS to -16, exported at 96kbps constant bit rate, mono, 44100 sample rate, in MP3 format via the LAME encoder.

What I hear is absolutely fluid sound, identical to the original files he has given me. He tells me that it is "metallic and barely audible." He sent a "test" file with my edited portion at the beginning, with his original file at the latter section of the file. I am unable to attach it as it comes up as "invalid file" here in the forums...

I tried it on a total of 6 other devices, and I am unable to hear even the slightest difference with my expensive mixing headphones. He says he sent it to 2 friends and they heard the exact same distortion...so I am stumped. He's an honest, straightforward guy. He's not some sociopath just trying to set me up for frustration, but everyone on my end hears consistency, and everyone on his end hears a metallic mess.

I have never encountered this before and I could really, REALLY use some insight into what might be happening

Thanks in advance!