Posting and answering this to help others experiencing the same issue as me. This was also asked here.
If only one keyboard is enabled, i.e. Google Keyboard - English (US), the keyboard shown whenever text input is needed sometimes does not have a microphone icon (or any option to select voice typing).
Some things that may not show:
- Microphone icon (in older Android versions, this was beside the space bar on the on-screen keyboard)
- Pressing and holding space bar does not trigger the input selection screen (that normally would have featured voice typing as an option).
Please refer to my own answer below https://android.stackexchange.com/a/104896/198