superelectrolizard ,
it is not software fault. your P8 having secondary mic (use for nice cancellation and voice and record sound with comparing. simply you can find it near the Headphone connector.
[guide|45850|Here] you can find teardown guide. check the mic contacts connected properly to logic board or replace the mic and see
as you mentioned on your question : not required to test primary mic as it works fine (bottom side mic)
Hope this answer will help you to understand the fault and fix it your self
Cheers!