do you have dither enabled? - noise shaped dither for 16/44 should give inaudible noise in any practical home listening scenario at normal listening levels
Audacity does do a simple frequency shaping of the additive dither but doesn't implement a good psychoacousitcally tuned shaped noise floor dither
any system will have a noise floor that you can turn up gain to unreasonable levels to hear - but you can't listen to music at those levels so it really isn't a good test
S/N numerical spec was gamed in some early some CD players - they have muting circuitry that detects "digital black" and shorts the output into a low R - much quieter than the player's DAC circuitry's electronic noise
many have come to see (hear) this output muting as artificial, pointless - since the noise floor with music playing is the only thing that really counts - and is almost always limited by the noise of the studio, microphones, electronics in the recording - few home listening rooms are as quiet as better pro recording studios too
