Try this
<audio controls="" accesskey="m">
<source src="jdc.mp3" type="audio/mpeg">
</source></audio>
But pressing Alt+"m" will focus the player only. You still need to press the space bar for play/pause.
Again, the key combination varies from browser to browser and OS to OS.
Check the list here-
HTML accesskey Attribute[
^]
However, you can write small javascript/JQuery code to do what you want.
Check following link for further details-
HotKey to Play/Pause HTML5 Music Player[
^]
Hope, it helps :)