i got my FP3+ replaced because the first model i got had annoying background noise on the side speaker and some 3.5mm wired headphones. You can read about it here: [FP3+] Hissing noise on speaker at low volume
This new model has less hissing, but still the audio on the side speaker and most wired headphones is pretty bad, especially on more quiet parts or silence in a track. If i listen to folk music on a low level, it sounds like the quiet parts get obscured by static noise.
Please note that this condition is the way it came with out of the box, with the latest updates installed.
There are some clues that it may be a misconfigured amplifier and/or missing software support for the audio hardware. In particular, if i use the Poweramp player with its optional high res audio output, the noise on the headphones is gone and its sharp and clear. The issues also do not appear on any bluetooth devices and are barely noticeable on wired headphones that are more quiet.
If you have some more in-depth knowledge on smartphone audio or experienced a similar issue, please let me know.