TYPO3 Core

At least in version 6.2.26, html5-audio is kind of broken.
The problem is simple: switching to audio hides the html5-option. Therefore you MUST select html5 (mmUseHTML5) BEFORE you switch to Type Audio.
Then the value mmUseHTML5=1 remains in flexform, although the checkbox isn't displayed anymore in backend.
In both cases, you could add HTML5-Audio files to the CE. ... although, if you did not select the option before, html5-audio won't get rendered ....
This is painful to explain to editors ...

Additionally I suggest to select 'HTML5' by default.
I failed to set it via PageTSconfig within 10 minutes, maybe I'm wrong, but it seems that this (setting flexform default values) cannot be done via the PageTSconfig TCAdefaults array.
This is what I tried successless:

TCAdefaults.tt_content {
  pi_flexform {
    media {
      sGeneral {
        mmUseHTML5 = 1
      }
    }
  }
}

The solution is rather simple.
Change this:

<mmUseHTML5>
  <TCEforms>
    <label>LLL:EXT:cms/locallang_ttc.xml:media.useHTML5</label>
        <displayCond>FIELD:mmType:!=:audio</displayCond>
        <onChange>reload</onChange>
        <config>
            <type>check</type>
            <default>0</default>
        </config>
    </TCEforms>
</mmUseHTML5>

to that:

<mmUseHTML5>
  <TCEforms>
    <label>LLL:EXT:cms/locallang_ttc.xml:media.useHTML5</label>
        <onChange>reload</onChange>
        <config>
            <type>check</type>
            <default>1</default>
        </config>
    </TCEforms>
</mmUseHTML5>