When you edited the Project, what level did you set for the Audio?
That it is good for some, but not for others, would likely indicate a setting in some of the playback instruments.
One can ONLY adjust for normal "standards," and if one has altered setting on their equipment, there is zero that a producer can do.
Also, when you go to the Window menu and select Audio Mixer, what audio levels show when you play your video on the timeline?
It could well be that you've just recorded at too low a level.
When I go to the window menu and select audio mixer, I get 0.0 at first. When I hit play, the greenish bar goes up and down as the people in the video speak, so the levels change constantly (sometimes -6, sometimes higher, sometimes less). I generally set the volume level at 0.0, but I raised the volume a bit and lowered it a bit at certain spots. The odd thing is that I can hear the voices on most devices, just not on an iphone with the headphones not plugged in. However, other videos work fine on the iphone without the headphone so that's why I don't think it's the device. Other video I've edited work fine on the iphone too, but maybe I made an odd editing mistake this time. I did use an external mic for the video, but I don't know if that should make any difference since I can hear the voices fine on the computer....