Here are the sentences changed into passive or active voice as requested:

1. **Active:** The first university in Vietnam is regarded as the Temple of Literature.
**Passive:** The Temple of Literature is regarded as the first university in Vietnam.

2. **Active:** The picture of the One Pillar Pagoda was bought by his father last year.
**Passive:** His father bought a picture of the One Pillar Pagoda last year.

3. **Passive:** The Hung Kings' Temple is visited by thousands of people every day.
**Active:** Thousands of people visit the Hung Kings' Temple every day.

4. **Active:** Who built the Imperial Academy?
**Passive:** By whom was the Imperial Academy built?

5. **Passive:** Khue Van Pavilion symbol can be seen on street signs in Hanoi.
**Active:** People can see the Khue Van Pavilion symbol on street signs in Hanoi.

6. **Passive:** Khue Van Pavilion is used as a symbol of Hanoi by Vietnamese people.
**Active:** Vietnamese people use Khue Van Pavilion as a symbol of Hanoi.

7. **Active:** What surrounds the Temple of Literature?
**Passive:** What is surrounding the Temple of Literature?

8. **Active:** UNESCO recognized Ha Long Bay as a World Heritage Site in 1994.
**Passive:** In 1994, Ha Long Bay was recognized by UNESCO as a World Heritage Site.