Place the player(s) in a div and give that a unique class:
Working example using two players:
<div class="audio_width_250px">
<strong>Silent Night</strong>
[audio mp3="http://swansonheritage.com/wp-content/uploads/2013/11/Silent-Night.mp3"][/audio]
<strong>Away in the Manger</strong>
[audio mp3="http://swansonheritage.com/wp-content/uploads/2013/11/Away-in-a-Manger.mp3"][/audio]
</div>
CSS:
.audio_width_250px {
width: 250px;
display: block;
}
Live at:
http://swansonheritage.com/christmas-favorites-by-lydia/