I’m having this problem as well except my situation is reversed — my right speaker’s volume is very low compared to the left’s. Tried resetting SMC and PRAM with no luck. Verified audio balance is in the middle. [guide|23430|Replaced my right speaker] with a “new” one to confirm it’s not a problem with the speaker itself. Moving my audio output balance over to the right is a hack that lets me enjoy me speakers at approximately the same volume again but this is obviously broken so I’m taking it to the Apple store tomorrow to see if they can diagnose the issue.