When I have my phone's ringtone volume dialled all the way down (vibrate only) and try to speak something with termux-tts-speak without specifying a stream, I get no sound.
Luckily, I discovered I could get audible speech when I up the ringtone volume a bit.

When I searched around, I found this SO question: What is the default audio stream of TTS?
The accepted answer states that the default stream for speech in Android is MUSIC, while termux-tts-speak uses NOTIFICATION. I later found confirmation in AOSP source code, please see TextToSpeech.java#283 in AOSP master

Please consider making MUSIC the default stream for speech, Thanks in advance!