I couldn't get that method to consistently work. I'm using first gen iPad Pro 12.9" running iOS 11.3 betaI think i found a quicker work around and a bit more about what's going on here. So I was trying your advise because when I sent the video to my computer it had no audio. So I was playing around with it and realised when I crop the original screen recording (on iphone) and "save as new clip" it actually saves as a .MOV file where as the original was saved as a .mp4. For some reason that let it have audio on my computer.
I have been dealing with this issue since the the first iOS 11 beta. My work flow requires uploading screen recordings to youtube for educational videos. The video plays fine on the iPad but when uploaded, the sound is lost. So I chalked it up to a youtube issue. This is not a youtube issue, take your screen recording and open it up in iMovie, the sound is gone there as well.
So that tells me this it is an apple issue, the screen recordings don't work properly in first party apps.
From what I gathered on various forums, there are two audio "channels" in the recording and when you send out the video to someone or upload, the player on the other end only plays one of the channels, which is usually silence.
The only consistent way I found to work, besides the method mentioned above about recording the recording, is to use an app called "Splice" its free and has some tie in with gopro
Use the app to create a project, pick your screen recording, skip all the fancy edits, turn off the ending slide, and then transcode the video to the resolution of your choice and save back to the device. I have confirmed that the audio is now present in all my youtube uploads.
Why has this bug gone on this long? Especially with the new push into education. I submit this bug every time a new beta comes out. I see that the 11.4 beta is available. I'm updating now and will report back if this is fixed or not in 11.4.
Edit: Audio bug is still in iOS Beta 11.4