Look at these 3 options:

As I mentioned before, the cost of living is high. [ACTIVE]

As mentioned before, the cost of living is high. [PASSIVE short form]

As was mentioned before, the cost of living is high. [PASSIVE full form]

Common mistake:

*The cost of living is high, as it mentioned by the speaker.

Correct: 

• …as mentioned by the speaker.
• …as was mentioned by the speaker.
• …as the speaker mentioned.