$ shellcheck myscript

Line 3:
find . -maxdepth 3 -iname '*.[Mm][Pp]3' -exec sh -c 'curfname="{}";\
                                                    ^-- SC2156: Injecting filenames is fragile and insecure. Use parameters.
>>                                                                 ^-- SC1004: This backslash+linefeed is literal. Break outside single quotes if you just want to break the line.

Line 4:
      bitrate=`mp3info -r a -p "%r\n" "$curfname"`;\
                                                   ^-- SC1004: This backslash+linefeed is literal. Break outside single quotes if you just want to break the line.

Line 5:
      if (( $(echo "$bitrate < 320" |bc -l) ));\
                                               ^-- SC1004: This backslash+linefeed is literal. Break outside single quotes if you just want to break the line.

Line 6:
      then \
           ^-- SC1004: This backslash+linefeed is literal. Break outside single quotes if you just want to break the line.

Line 7:
         echo $curfname has $bitrate bitrate; \
                                              ^-- SC1004: This backslash+linefeed is literal. Break outside single quotes if you just want to break the line.

$