WordPress audio player causing js error, mediaelementplayer is not a function

I can think of two possible options – although I am not been able to test either right now.

Option 1

In theory the cleaner one, as you detect if the mediaelementplayer plugin has loaded itself in the jQuery namespace:

<!--Change WordPress Audio Player Default Volume-->
<script type="text/javascript">
jQuery(document).ready(function ($) {
    if($.fn.mediaelementplayer) {
        $("audio").mediaelementplayer({
            success: function (mediaElement, domObject) {
                mediaElement.setVolume(1.0);
            }
        });
    }
});
</script>

Option 2

If Option 1 doesn’t work, then this should; it tests the current jQuery object to see if it responds to the mediaelementplayer call:

<!--Change WordPress Audio Player Default Volume-->
<script type="text/javascript">
jQuery(document).ready(function ($) {
    var audio_widgets = $("audio");
    if(audio_widgets.mediaelementplayer) {
        audio_widgets.mediaelementplayer({
            success: function (mediaElement, domObject) {
                mediaElement.setVolume(1.0);
            }
        });
    }
});
</script>

Hops this helps!