The sentence has a small grammatical error due to the quotation marks not being closed properly. It should be corrected to:
“Peter joined me in this ‘ancient oja’ sound.”
Rephrased, it could be:
“Peter participated with me in this sound referred to as ‘ancient oja.’”
The meaning of the sentence suggests that Peter and the speaker were together, and they were engaging with or experiencing a sound described as "ancient oja." The