The sentence "I will be working as a volunteer next summer" is in:
- Tense: Future
- Aspect: Progressive (also called Continuous)
- Mood: Indicative (not Subjunctive).
The subjunctive mood expresses wishes, hypotheticals, or conditions contrary to fact. For this sentence to be in the subjunctive mood, it might look like:
"If I were to work as a volunteer next summer, I would gain valuable experience."