This code (jQuery) works for me in iPhone Safari iOS7 for continuous playback. Even if I have my screen locked/other tab it changes track when the first has ended (given a bit of patience for the new track to load)
<audio id="audio1" src="song1.mp3" controls></audio>
<script type="text/javascript">
$('#audio1').on('ended',function(){
$(this).attr('src','song2.mp3');
$(this)[0].play();
});
</script>
I've read has suggested that Safari prevents audio from playing unless it's initiated by a user.
=> That is true and is by design - no work around known as of today (well ... I have seen ugly attempt to make it work).