If it happens that you hear the sound in one channel only on particular systems, like a headphone, it is possible that the song is originally fine, but who converted it set the output to mono. And different systems treat mono recording differently. Some automatically makes it stereo, others are set to project the sound into a desired channel, left or right. I guess those people who has problems on headphones, when they plug it into their computer, the computer driver detects the headphone, and possibly the driver is set that in case you plug in a headphone, it sends the signal to one channel. Check your audio preferences on the computer and set both for speaker system and headphone that a mono recording should be projected to both channels than to only 1. It is in the preferences of your operating system.