Apple changed the default with watchOS 3, likely with the goal of making the interaction faster (and more people probably used voice-to-text), but you can easily change it back to restore the option or always send as a voice recording.
How to send voice messages from Apple Watch
Start by launching the Watch app on the iPhone. From the My Watch tab (the first tab), swipe down to the Messages section.
Tap to reveal options for Messages on Apple Watch including a section called Dictated Messages. Tap this option to change the default ‘Transcript’ option to ‘Audio’ or ‘Transcript or Audio’ depending on your preference.
Here’s how Apple describes the default option on watchOS 3:
By default, when you’re done dictating a message, Apple Watch sends a written transcript of what you said. You can change this setting to send an audio clip instead, or to choose which option you want every time you dictate a message.
Personally, I prefer the option to send either the transcript or audio. I sometimes dictate a message using voice-to-text on my Apple Watch, but I also enjoy using my Apple Watch as a sort of walkie-talkie to send quick voice recordings to friends and family. (Also, my dictation voice is much more formal than my voice recording voice.)
If you never send voice-to-text dictated messages, however, you can use this section to only send voice messages every time and save yourself the step of selecting the audio option every time.
Once you select your preferred option, give it a try on the Messages app for Apple Watch. Look for the microphone button when replying to a new message, then speak away. The recipient can then playback the audio over iMessage from Messages for Apple Watch, iPhone, iPad, iPod touch, or Mac and respond with text or voice … walkie talkie style.