I also assume 32-bit audio corresponds to the bits per sample.
1 << 31 is a left shift by 31 positions, so it translates to 1 << 31 == 2**31  == 2147483648, which would be the max value of each sample.
If I’m not mistaken, 32bit audio would have the range [−2,147,483,648, 2,147,483,647], so you would get a minimal error for the max positive value.