Q:

How to pull a single podcast source, using the Matrix field and loop? 

{% if block.type == "podcastBlock" %}
    {% for category in block.podcastChannel %}
        {{category.title}} 

        <audio class="mp3" controls>

        {% for entry in block.podcastChannel %}
            <source src="{{ entry.audioFile.one.url }}" type="audio/mp3">
        {% endfor %} 
    {% endfor %}
{% endif %}`

A:

The issue was using two loops.  I just needed the initial loop and referencing the loop call with the field-handle together.  It now retrieves a single podcast. 
{% if block.type == "podcastBlock" %}
    {% set groupTitle  = craft.categories.group('podcasts').one() %}

    {% for podcast in block.podcastChannel %} 
        <p class="subhead-section">{{ groupTitle }}</p>
        <audio class="mp3" controls>
        <source src="{{ podcast.audioFile.one.url }}" type="audio/mp3">
     {% endfor %} 
{% endif %}

