Is it possible to record a voice-memo in iPhone or in MacOS, and to see/play that recording file from within the Apple Watch?
Yes, by manually sending the recording to your Apple Watch via Mail
- On your iPhone, open the (Apple) Watch app.
- Go to My Watch > Mail > Include Mail.
- Now, go to My Watch > Mail > Accounts and select the account.s on which you want to send your recording (the one.s you want, doesn't matter).
- Then, once this is set up, open the Voice Memos app on your iPhone.
- Click on the recording (the name of it) you want to listen on your Apple Watch, it should show the play bar and the play button as well as some other options.
- Then, click on the three-dots at the top-right-corner of the recording, this should open a "Share"-like menu.
- Now, choose either (1) Save to Files, (2) Copy, (3) Share.
8.1. The following steps will be if you chose (1) Save to Files.
8.1.a When you click on Save to Files, it should show a pop-up window asking you where you want to save the file (as .mp4), choose the place where you want it to be and tap "Save" (top-right-corner).
8.1.b Now, still on your iPhone, open Mail app and compose an email to the associated email address (the one you associated to your Apple Watch).
8.1.c For the email body, put whatever you want but at some point, click on the document symbol (on top of the letter "i" on your (built-in) keyboard), this should make the same pop-up window with Files view.
8.1.d Choose the file you've just imported to Files, it will automatically include it in the email body.
8.1.e Once, you're happy with your result, tap on send.
8.2 The following steps will be if you chose (2) Copy.
8.2.a Once you have clicked on Copy, the file (.mp4) will be copied to your clipboard. Open Mail app.
8.2.b Now, still on your iPhone (in Mail), compose an email to the associated email address (the one you associated to your Apple Watch).
8.2.c Then, click on the cursor (blue pointer indicating where you are typing), it should show a menu on top of it, click on Paste. This should paste the whole .mp4 file into the email body.
8.2.d Once, you're happy with your result, tap on send.
8.3 The following steps will be if you chose (3) Share.
8.3.a Once you've tapped on Share, a menu proposing apps will appear in the same dialog window (where the options are). Click on Mail.
8.3.b It should open up a composing email with the recording (as .mp4 file) in the body. Write the associated email address (the one you associated to your Apple Watch) as the person you're sending the recording to.
8.3.c Once, you're happy with your result, tap on send.
Once this sharing by Mail process is done, on your Apple Watch, open Mail app.
(You may need to reload the inbox to see the email with the attachement) Open the email and tap on the .mp4 file (in the body).
Now, you can press play and listen to it.
Note : you can also skip steps
8.1.d by : (Opening Mail app, clicking on the compose button - that's in step
8.1.b) going to Files app and click-and-hold on the .mp4 file you've just imported, then (keep you finger pressed), with another finger, re-open Mail and dropping the file in the body of the email being composed.
Note 2 : For more information about steps
8.1.d, I propose you read this Apple Support article about how to Add email attachments in Mail on iPhone.
Note 3 : This answer is assuming you are using Apple Watch Ultra running watchOS 9. This is sure to work on that version but it is highly probable to work on oldest ones (watchOS 6-later).
For more information about using Mail in Apple Watch, I suggest you consult this Apple Support article.