Audio clip properties: channels bit_depth samplerate duration

### 15.1.6 Audio clip properties: channels bit_depth samplerate duration

The channels, bit_depth, samplerate and duration commands find properties of an audio clip.

- channels, bit_depth, samplerate and duration each take one argument:
  A, an audio clip.
- channels(A) returns the number of channels in A.
- bit_depth(A) returns the number of bits reserved for each sample value (8 or 16) in A.
- samplerate(A) returns the number of samples per second in A.
- duration(A) returns the duration of A in seconds.