I am trying
[audio mp3=\"https://abcd.com/wp-content/uploads/sites/2/2020/03/classical-demo.mp
I'm not PHP
developer, but I think you can use following code:
$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;
exit();
$b
will be: link text
See it in action thanks to @Phil