Replace a string in directory name using a variable
Solution

Replace your line:

base="$(basename $str)"


with:

base="$(basename $str | sed 's/FLAC/V0/')"


or, if you want the [ and ] brackets to become part of the search pattern:

base="$(basename $str | sed 's/\[FLAC\]/[V0]/')"


Explanation


The | (or pipe) character essentially means “take the result and feed it into the next program”.
The sed (stream editor) command takes the input, applies commands to it, and prints the result. For example, the s/FLAC/V0/ command in sed means “for each input line, search for FLAC and replace it with V0 (but no more than once)”.
sed uses regular expressions instead of fixed search patterns. That means you need to escape certain characters; hence, my last example says \[ and \] in the search pattern (instead of just [ and ]).