I could not reproduce your results. Your example output an empty attribute in my test code. Please provide a complete program with your actual code and output.

use warnings;
use strict;
use HTML::Tiny;

## from the module documention
my $h = HTML::Tiny->new( mode => 'html' );
print $h->closed( 'input', { type => 'checkbox', checked => [] } );
print "\n";

## example from OP
print $h->tag ('audio', { controls=>[], type => "audio\/mpeg", src => 
+"test.mp3"});

__DATA__
<input checked type="checkbox">
<audio controls src="test.mp3" type="audio/mpeg"></audio>
[download]