There is no extra slider for TTS voice in program.
You can increase TTS voice level in Android settings - Text-to-speech - Vocalizer TTS settings.
Increase "WAV amplitude" from default 80 to 100, which will give you additional ~2 dB.

You can also try to change audio stream in sys.txt:
[android]
; SELECT AUDIO STREAM:
; 0 - STREAM_VOICE_CALL
; 1 - STREAM_SYSTEM
; 2 - STREAM_RING
; 3 - STREAM_MUSIC
; 4 - STREAM_ALARM
; 5 - STREAM_NOTIFICATION
; 6 - STREAM_BLUETOOTH
; 8 - STREAM_DTMF
navigation_audio_stream=4
And then in Android try to increase volume for that sound, in this example it is "ALARM".
I had best results with values 4 or 5 (NOTIFICATION).