I uploaded a backing track into Audacity and then recorded vocals. Everything sounds fine on computer speakers or headphones, but the vocals can’t be heard on my iphone when playing mp3. Any way to fix that? Thanks!
go the same prob!
Somehow, the left & right channels for the vocals are out-of-phase (one channel is inverted relative to the other). When they are mixed to mono the left & right signals cancel.
So the first question is how did that happen… Tell us about your hardware setup. i.e. What kind of microphone are you using and how is it connected? Did you apply any effects?
If you have a vocal-only track it can be fixed by selecting either the left or right channel and inverting it. If you only have a vocal & music mix, inverting one channel will bring back the vocals but it will cause issues with the stereo background music.
I am using a Shure Super 55 Supercardioid dynamic microphone. It is connected to the PC microphone port with a Cable Matters XLR to 3.5mm Female to Male Stereo Microphone Cable. I don’t know anything about the computer soundcard…
I will try inverting one side of the vocal track when I get home from work. 
Oh, and I applied the following effects: noise reduction, soft limiter, and amplify to the vocal track
Thanks