In browsers supporting <audio> (e.g. Firefox 3.5, Chrome), the controls appear too large, breaking page layout. In Firefox playback starts with an annoying "buffering" animation that temporarily makes the controls higher, further breaking layout.
Here is a good example:
Or here, in the infobox:
The size of this control bar can be set with CSS:
<audio style="width:100px;height:28px" src=...></audio>
(works in Chrome and Firefox at least)
I recommend the 28px height because it looks fine (the controls are not clipped) and the FF buffering animation is hidden.