YOU SAID:

i was on a audition yesterday

INTO JAPANESE

私は昨日オーディションに参加していた

BACK INTO ENGLISH

I was in the audition yesterday.

INTO JAPANESE

私は昨日オーディションに参加していました。

BACK INTO ENGLISH

I was at the audition yesterday.

INTO JAPANESE

私は昨日そのオーディションにいた。

BACK INTO ENGLISH

I was at the audition yesterday.