Specific MP3 plays over headphones only

Is it your MP3?
Is it your website?

If audio is dual-mono, but in anti-phase, it will be audible on headphones, but when mixed down to mono,
e.g. when played on a device with a single loudspeaker, (like some phones), there will be silence, (or just some faint bubbly compression-artefact noise).

If that it the cause of the problem, deleting (or inverting) one of the dual-mono pair is the cure.