How to get related entries from a Matrix block itself related to another section
You access the wrong variable to get to you "compDiscographie" Matrix field. It should be:

{% for block in record.compDiscographie %}


Another thing to change here is to add the type parameter, as you probably have other Matrix block types in that field (if not, why did you choose Matrix?):

{% for block in record.compDiscographie.type('myPerformersBlockTypeName') %}


Then you should be good. Oh, and you better have that &lt;ul&gt; outside your loop!