The Audio Board with my Teensy 3.2 is working fine... but it's to quiet.
If I make it louder (Volume > 0.5) then I hear only sound cracklings, so Volume: 1.0 is not possible

Short files < 300 KB you can not use, because you her sound cracklings only.

How I can copy the SD wave file from the SD Card into the memory, to play it as sample sound?

And how I can make a sound louder? (Mixer -> Loudness ?) or can I normalize a loaded sound?