Unlike the character Devalem, who has one voice, Argo has two: One for telepathic communication, and the other for singing, as a Concordian's physical voice is too powerful for mortal ears to hear if they lack the skill to control it.

For key reference, use Chris Cornell, who was the lead singer and rhythm guitarist for the rock bands Soundgarden and Audioslave. Of course, it's impossible to find an exact match, so try your best to get somewhere around that vocal sound, but not to the point where it becomes an impersonation or impression. Compared to Chris Cornell, Argo's range is on the lower side. Feel free to add a backing/karaoke track to your audition.

Here is a link to 4th Of July by Soundgarden.