After installing a ssd into my Macbook pro mid 2012 (not retina), i noticed that the startup chime is coming only from the right speaker. (I am not sure if it was always like that or if its because of the installation). However when I am in the OS, both speakers work perfectly fine. I reseted the NVRAM a couple of times already. Is the chime supposed to come out of the right speaker only?
I am pretty sure your MBP always did that. It is something annoying I never really liked about the MBP: They have a "subwoofer" which is situated on the right-hand side of the MacBook. So sounds with a lot of bass always appear to come from the right speaker. Here is what Apple says.