You might be confusing bitrate and sample rate.
'320' is a number usually associated with bitrate. 320kbps is one of the bitrates available using MP3 compression which would be typical for Spotify content.
Sample rate is the number of sound samples per second. For Red Book CD Audio, it is 44,100sps or 44.1ksps. Most of the music on Spotify will be at this sample rate.
Red Book CD Audio has 16 bits per sample, hence 44,100 x 16 = approx. 1400kbps. The 320kbps of MP3 is quite severely compressed (and that's the best that MP3 has to offer) but will be uncompressed by your player and sent as 44.1ksps/16bit to the DAC.