is there a program that does text and voice over xmpp?

Pidgin can make voice and video call via xmpp/jabber only if your and
your friend's xmpp service provider allows it.

Cool, thanks.

Do you know wheter Jabber.org is one of those (video-supporting) service providers?

Pidgin does text, voice, and video over xmpp.

The Windows-port of Pidgin does only text. So if you want to set up a voice chat with a friend over Pidgin and your friend uses Windows, it will fail.

For a long time it was unclear to me what the problem was, until I saw Pidgin running in Windows.

Have you tried Jitsi? [1]
I can really recommend it because it knows how to handle text, voice and even video. (encryption works as well)

[1]: https://en.wikipedia.org/wiki/Jitsi

Is there a way to have voice conversations with xmpp, between Trisquel and Android?

You can try Linphone (SIP) and Jitsi (SIP and XMPP).
Linphone works with Android.

In Android you need one free software with audio and video support (Jingle) for XMPP.

Empathy does XMPP/Jingle pretty well.

The only free software client for Android that I know of which supports Jingle is Jitsi. And that is still in alpha stage.

Linphone which was suggested above is SIP. Jitsi supports SIP as well. Another great SIP client for Android is CSipSimple.

If you want only text chat then there are more free software XMPP clients for Android: Beem, Gibberbot, Xabber, Yaxim etc.

You can search the f-droid database to find free software Android apps: http://f-droid.org