Ok, credo di aver risolto!
Il codice che filtrava solo gli mp3 e i flv era qualche riga più sopra:
Originale<td class="quickCol">{.switch|%item-ext%|,|
jpg,jpeg,png,bmp,tif,tiff|
{.if|{.filesize|%folder-resource%\previews_and_thumbnails\prev-%item-name%.jpg.}
|{.$preview-btn.}
|<div class="quickBtn disabled hide"><img src="/template/{.!theme.}/images/preview.gif" alt="" align="middle"/></div>
.}
|mp3,swf,flv|{.$preview-btn.}|<div class="quickBtn disabled hide"></div>.}
<noscript><div class="quickBtn disabled"></div></noscript>
<div class="quickBtn1 {.if not|{.get|can access.}|disabled.}"><a href="%item-url%" {.$target.}><img src="/template/{.!theme.}/images/download.gif" alt="DL" title="{.!lv_downloadTitle.}"/></a></div>
</td>
</tr>
[preview-btn]
<div id="s%sequential%" class="quickBtn hide" {.switch|%item-ext%|,|mp3,flv|name="mediaFile"|.}><a href="javascript:void(0)" onclick="initPreviewBox('Box-'+(%sequential%+1),this.name)" name="%item-full-url%"><img src="/template/{.!theme.}/images/preview.gif" alt="P" title="{.!lv_previewTitle.}" align="middle"/></a></div><span id="Box-%sequential%"></span>
Modificato<td class="quickCol">{.switch|%item-ext%|,|
|
{.if|{.filesize|%folder-resource%\previews_and_thumbnails\prev-%item-name%.jpg.}
|{.$preview-btn.}
|<div class="quickBtn disabled hide"><img src="/template/{.!theme.}/images/preview.gif" alt="" align="middle"/></div>
.}
|mp3,swf,flv,avi,mpg,mpeg,m2p,wmv|{.$preview-btn.}|<div class="quickBtn disabled hide"></div>.}
<noscript><div class="quickBtn disabled"></div></noscript>
<div class="quickBtn1 {.if not|{.get|can access.}|disabled.}"><a href="%item-url%" {.$target.}><img src="/template/{.!theme.}/images/download.gif" alt="DL" title="{.!lv_downloadTitle.}"/></a></div>
</td>
</tr>
[preview-btn]
<div id="s%sequential%" class="quickBtn hide" {.switch|%item-ext%|,|mp3,swf,flv,avi,mpg,mpeg,m2p,wmv|name="mediaFile"|.}><a href="ums://TCP:127.0.0.1:5119%item-folder%%item-name%"
onclick="ums://TCP:127.0.0.1:5119%item-folder%%item-name%"><img
src="/template/{.!theme.}/images/preview.jpg" alt="P" title="{.!lv_previewTitle.}" align="middle"/></a></div><span id="Box-%sequential%"></span>
Naturalmente è necessario avere installati tutti i codec necessari sulla macchina client da cui si lancia la preview