<?php
$a = '[audio mp3="https://abcd.com/wp-content/uploads/sites/2/2020/03/classical-demo.mp3"][/audio]';
$b = str_replace("[","<",$a);
$b = str_replace("]",">",$b);
$b = str_replace("audio","a",$b);
// you need to replace mp3=, not mp3, as you have 2 of it
$b = str_replace("mp3=","href=",$b);
// optional
$b = str_replace("><",">link text<",$b);
echo $b;