I got a “new” (second-hand) laptop (an old HP Probook) last month, and installed Debian on it, and I use it for all of my writing and much of my music-listening, but I’ve encountered a weird issue and I’m wondering if anyone has any advice on this.
If I listen to music with headphones, the centre channel cancels out. To listen to, say, the new mix of Sgt Pepper with headphones, I have to unbalance the left and right channels so they no longer go out of phase.
EXCEPT that with one pair of headphones — annoyingly a really bad set of earbuds which don’t fit my ears very well — this doesn’t happen. So it can’t be a headphone port issue. The headphones which don’t work on my laptop work fine on my portable MP3 player, so it can’t be an issue with the headphone wiring. These are all cheap earbuds without any fancy Beats-style additional software, so I can’t see what could be causing this.
My best guess, given the general flakiness of sound on GNU/Linux, is that it’s an issue with PulseAudio or ALSA somehow — but how that could only be affecting some headphones and not others, I have no idea. Anyone at all have a clue?
Sci-Ence! Justice Leak! 
They all have 3.5mm stereo jacks?
I don’t see how PulseAudio could cause it, but I’ll blame it anyway. PulseAudio fixes all your Linux sound problems, except ones you have.
Yep, all identical, standard, 3.5mm jacks. All work fine in any other devices.
And yeah, I have no idea at all how Pulse could cause it, but I bet it does.
Triage: The really bad set of earbuds is definitely producing stereo and not 2 channels of mono? You’ve tried killing pulseaudio and having your player talk directly to ALSA? Are there any interesting advanced settings in alsamixer (e.g. surround sound)?
Definitely stereo. I’ll try those other ideas — I’ll have to get alsamixer installed first though.