Firstly, download and install an iPhone manager tool on your PC, there are many tools can do this, such as free Syncios Manager. Install it on your PC. And then connect your iPhone to your PC with a USB cable.
Secondly, click on Media tab on the left of the panel after your iPhone gets connected with the manager tool. And find Voice memo options on the right top of the panel. Select the very voice memo which you’d like to convert into a ringtone, export it to your desktop or other folder on your PC.
Lastly, click on the Toolkit option, you will see massive handy tools there, such as ringtone maker, audio converter, video converter And GIF maker, etc. To convert the ringtone, click on Ringtone Maker button on the Common Functions, simply select the Voice memo you just exported on your desktop. After that, you can convert it into the ringtone. you are able to save the ringtone on your PC or transfer the ringtone to your iPhone directly by clicking on the Save button below.
With all the steps done, navigate to your iPhone, open Settings>>Voice>>Ringtone, select it to set it as your ringtone.