maybe I'm missing something but my audio-levels with the built-in mics are very low although gain turned up to 100 %. And when an external mic is plugged-in via the 3.5mm-input there's barely any level - in fact only if I hit the mic and the gain-sliders are greyed out for that input. The mics, Rode Stereo Video Mic Pro and Tascam TM-2x, do work on the BMPCC. I don't have an adapter at hand to test the XLR-input.
You should try to select the 3.5mm mic input, which doesn't show up in the cycle until you either unplug everything or plug in the mic level inputs. You may also need to turn off and on. The whole operation was hit or miss. If the level is still too low you may either need a mic with more output , perhaps with it's own phantom power or an external phantom power supply. With XLR the camera can provide phantom power.
The camera prevents you from selecting a different input level on the 3.5mm option if one is already selected. For example, if you have 3.5mm Left - Mic set as your Channel 1 Source, you cannot select 3.5mm Right - Line as Channel 2 Source.
If you need to change to a different input level, make sure you switch both channel sources to something else before selecting the correct input source.
There is no gain adjustment available for Line level inputs.
I just tried the camera for the first time and I had the same problem. Hadn't selected anything before, but had to plug, unplug and reboot to get to the 3.5mm MIC option.
I experienced the same thing, took me coming on here reading this thread, then I had to unplug my Sony wireless, plug back in and turn on/off camera to see the 3.5 mic option. Very disappointed with the sound though as I had to turn the again up to 80-100% and also lower my attenuation to -0db and even then the audio was recoding at around -24db to -18db with background noise from the raised gain level.
Am I missing something here? Is the mini-XLR port any better?