Improve the bracketed part of the sentence.
It was so hot during the concert that some spectators passed (out).

Let's look at the meanings of the phrasal verbs:
pass out: to become unconscious; faint. This perfectly fits the context of extreme heat during a concert causing people to faint.

pass away: to die.

pass by: to go past something or someone without stopping.

pass over: to ignore or overlook something.

Thus, the underlined part is completely correct. No improvement is needed.