Here are the corrections for the mistakes found in the text:

1. **The company has recently began**:
**Correction**: The company has recently **begun** implementing new strategies to improve its efficiency.

2. **I have already drunk**:
**Correction**: I have already **drunk** three cups of coffee this morning.
*(Correct as "drunk" is used correctly. However, "had" could be used for past perfect if referring to a specific past event.)*

3. **He has received many compliments on his new shoes since he has bought them**:
**Correction**: He has received many compliments on his new shoes since he **bought** them.

4. **The community has recently came together**:
**Correction**: The community has recently **come** together to support those in need after the disaster.

5. **I just have finished a relaxing yoga session**:
**Correction**: I just **finished** a relaxing yoga session to reduce my stress level.

6. **Have you already worked on this project since three years?**:
**Correction**: Have you already worked on this project for three years?

7. **Robots have developed the ability to dream or imagine yet.**:
**Correction**: Robots have developed the ability to dream or imagine **not yet**.

8. **I hadn't heard from them since they changed their phone numbers.**:
**Correction**: I haven't heard from them since they changed their phone numbers.

9. **Have you ever learned how to play a musical instrument before?**:
**Correction**: Have you ever learned how to play a musical instrument?

Please review to ensure clarity in context, as some corrections depend on the exact meaning intended.