Audactiy is a free, easy-to-use audio editor (with powerful advanced options that are more complicated to use). Audacity is probably the best free option for WINDOWS. (There are versions for MAC and LINUX. But MAC users can easily create mp3 files using "Garage Band.") Audacity is great for teachers and particularly for teachers of ELLs. Download from here.
This tutorial, from January 2022, will walk you through the process of downloading and installing Audacity.
This explanation, from June 2022, is quite good for the basic functions. This will give you more than enough information to start with. Btw, you can ignore the promotion for the advanced tools in the first 31 seconds of the clip.
This short tutorial, from September 2021, demonstrates how to create fade-ins and fade-outs. This is particularly useful (and cool) when you want to have music before and/or after your voice. Record your voice on a new track and then fade-out the intro music and use the "slider" option to start your voice before the music actually disappears. At the end of your voice recording, you can create a fade-in for the music to appear under your voice. Once again, you can use the "slider" option to get the combination of the two tracks just right.
For more advanced tutorials about Audacity, click here.