If you've got the audio_image module enabled you can attach images to audio nodes. Currently the images are not displayed as part of the teaser. The following code snippet shows how to override the default teaser theme function on a PHPTemplate theme. Add this to your template.php file.

For Drupal 5

/**
* Format the teaser for an audio node.
*/
function phptemplate_audio_teaser($node){
  // make sure that all the allowed tags are included.
  foreach (audio_get_tags_allowed() as $tag) {
    $params['!'. $tag] = isset($node->audio_tags[$tag]) ? check_plain($node->audio_tags[$tag]) : '';
  }
  $params['!filelength'] = theme('audio_format_filelength', $node->audio_fileinfo);
  $params['!fileformat'] = theme('audio_format_fileformat', $node->audio_fileinfo);
  $params['!player'] = audio_get_node_player($node);
  $params['!play_count'] = check_plain($node->audio_fileinfo['play_count']);
    $params['!download_count'] = check_plain($node->audio_fileinfo['download_count']);
// this is start off edit
    $params['!images']= theme('audio_images', $node->audio_images);
//and of edit
  $format = variable_get('audio_teaser_format', '!player<br />!filelength');

  return t($format, $params);
}

For Drupal 4.7

function phptemplate_audio_teaser($node) {
  // make sure that all the allowed tags are included.
  foreach (audio_get_tags_allowed() as $tag) {
    $params['%'. $tag] = isset($node->audio_tags[$tag]) ? $node->audio_tags[$tag] : '';
  }
  $params['%filelength'] = theme('audio_format_filelength', $node->audio_fileinfo);
  $params['%fileformat'] = theme('audio_format_fileformat', $node->audio_fileinfo);
  $params['%player'] = audio_get_player($node);
  $params['%play_count'] = $node->audio_fileinfo['play_count'];
  $params['%download_count'] = $node->audio_fileinfo['download_count'];

  // THE CHANGE FROM THE ORIGNAL FUNCTION IS HERE
  // if there's an image, put it in as an %image parameter.
  if ($image = audio_images_get($node->audio_images)) {
    $params['%image'] = "<div class='audio-image'>\n" . theme('audio_image', $image) . "\n</div>\n";
  }
  // THAT'S ALL

  $format = variable_get('audio_teaser_format', '%player %filelength');

  return t($format, $params) . $node->teaser;
}

For more info see #78883, the issue that spawned this.