Fix MixerTTS data loading index error
Signed-off-by: Jocelyn Huang <jocelynh@nvidia.com>

# What does this PR do ?

Fixes indexing error in MixerTTS-X data loading introduced by adding items to the TTS datasets

**Collection**: TTS

# Changelog 
- Changed indexing from 13 to 15 due to two additional items `voiced_mask` and `p_voiced`

**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation