Concerning Bit Rate, what is the minimum that could be used on an HD timeline? If the audio is at 48,000HZ, is 16 Bit rate audio possible? Does it have to be 24 or 32 for it to be considered HD?
HD has never meant anything other than a video size frame bigger than SD.
Accepted audio rates and bit depths can include any of the following:
for SD material on DVD
16bit - 48kHz
16bit - 96kHz
24bit - 48kHz
24bit - 96kHz
for HD material on Bluray
16bit - 48kHz
16bit - 96kHz
16bit -192kHz
24bit - 48kHz
24bit - 96kHz
24bit - 192kHz
32bit does not exist as a delivery format, it's the accuracy at which internal calculations are performed.
It is unlikely you will see a noticeable improvement in your audio if you render out to a higher setting than what your source material is. But in professional mixes it will give more room for dynamics. Your audio will be cleaner sounding at low levels because there is less quantization error.
Almost all cameras still record at 16bit - 48kHz
except for Red Cam - 24bit or DigiBeta -20bit
Most likely you should just leave everything at 16Bit - 48kHz for compatibility sake. For example: Windows media player cannot play 24bit wav files.