wav files loudness depend on the ampliture of the signal.
Loudness is the highest possible when they range from -1 and +1. If range is lower the sound will be quiter, if higher waves will be truncated and you'll hear a "scretchy" sound.
Try scaling your signals like that so as to fit the -1 / +1 range.
Furthermore, frequencies must be in the udible spectrum (technically 20 Hz - 20,000 Hz, but intermediate frequencies are listened more easily by our ears).
But try scaling the signal first, and let us know if you still have issues.
