Hi!

First of all, I apologize for my awful english.
Second of all, I'd like to know how can I add 3 long duration audios (more or less 17 min each one) and assigned them to 3 different experimental groups?
I've been trying with SOUND element, but honestly I haven't been able to understand it.
At least, I'd like to know if it is possible to add such a long duration videos.

Thanks in advance.
Regards,

If the audio files are in WAV format, use the <sound> element. If the files are in some other, compressed format use the <video> element. The length is not an issue. The rest depends on what exactly you want to *do* with the audio. <sound> and <video> work like any other stimulus element (<text>, <picture>, etc.), i.e. you "display" them via a <trial>'s /stimulustimes or -frames, or via a <block> element's /bgstim.

As for assingment to three experimental conditions, create three <expt> elements -- one for each condition -- and then do something like

<values>
/ itemnumber = 1
</values>

<video audio>
/ items = audioitems
/ select = values.itemnumber
...
</video>

<item audioitems>
/ 1 = "audio_a.mp3"
/ 2 = "audio_b.mp3"
/ 1 = "audio_c.mp3"
</item>

with

//1st condition
<expt>
/ subjects = (1 of 3)
/ groupassignment = groupnumber
/ onexptbegin = [values.itemnumber = 1; ]
/ blocks = [1=myblock]
</expt>

//2nd condition
<expt>
/ subjects = (2 of 3)
/ groupassignment = groupnumber
/ onexptbegin = [values.itemnumber = 2; ]
/ blocks = [1=myblock]
</expt>

//3rd condition
<expt>
/ subjects = (3 of 3)
/ groupassignment = groupnumber
/ onexptbegin = [values.itemnumber = 3; ]
/ blocks = [1=myblock]
</expt>

<block myblock>
/ bgstim = (audio)
...
</block>

Thanks for your quickly answer, it was really useful. Nevertheless, I'm still having problems with my script.
I am editing a Serial Reaction Time Task, in which I put 2 different secuences (circleblock) that is expected to run depending on the subject number, if it is odd then circleblock1 will run or if it is even, circleblock2 will run. 
Also, my experiment will have 3 experimental groups that depend on  3 different meditation audio types (fam, omm and control), and I expecte that  the group number will be aligned with one of those audios. (This is the parte with which I have problems)
 I attach the script with which I am working.

Thank you so much!

See attached.

It works perfectly, thanks.
Just to know, what does the zero means in the next part of the script?
/ onexptbegin = [if (mod(script.groupid, 3) == 1) values.itemnumber = 1;]
/ onexptbegin = [if (mod(script.groupid, 3) == 2) values.itemnumber = 2;]
/ onexptbegin = [if (mod(script.groupid, 3) == 0) values.itemnumber = 3;]