What are some potential pitfalls of using a visual image-based alternative to Captchas in PHP for spam protection?

One potential pitfall of using a visual image-based alternative to Captchas in PHP for spam protection is that visually impaired users may have difficulty completing the task. To solve this issue, you can include an audio option for users to listen to a code instead of having to visually identify it. 

// Generate a random code
$code = substr(md5(rand()), 0, 6);

// Save the code in a session variable
$_SESSION['captcha_code'] = $code;

// Create an audio file with the code
$text = "Your code is " . $code;
header("Content-Type: audio/wav");
header("Content-Disposition: attachment; filename=captcha_code.wav");
echo text2wav($text);

function text2wav($text){
    $text = escapeshellarg($text);
    $file = tempnam(sys_get_temp_dir(), 'captcha_').'.wav';
    exec("espeak -w $file $text");
    return file_get_contents($file);
}

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