Digital audio is a simple, engaging way to have you and your students hear each other's voice over the internet. Audio recording can impart meaning and tone into one's presentation that is otherwise more challenging to convey in text. Students can gain experience and confidence in recorded or even public speech. It can add a degree of warmth and community within an online course, since class members hear human voices, instead of simply know each other through the typed word. And unlike video (discussed below), audio does not require someone to present themselves visually, which may be more comfortable and less time-consuming. So it's a good balance between adding more personality to the course, without creating additional stresses.
Both Windows and Mac have simple audio recording applications built-in, and more advanced tools are available for editing audio. Examples include Audacity, or Garageband for the Mac.
A form of internet audio presentation, "podcasting" appeared in the early 2000s, as the web was gradually able to handle more data, but before video could be efficiently streamed. Podcasts are usually serialized programs, akin to radio shows, but are basically downloadable or streamable digital files, playable on demand. If you have never had exposure to podcasts, the best thing is to find a series you like and listen to it.
You might consider creating a series of podcasts for your course that are a series of lectures, weekly summaries, or just basic instructions. These are handy for students who can listen to them on their mobile devices, for example while commuting or exercising. And unlike a face-to-face course, students can relisten to them in whole or part, any time they want. You can host audio files generally in D2L, but if you wish podcasts to be available outside the LMS (and to people not enrolled in your course) you can host or upload them to the web in various places. One example is Podbean.com
However, (as with video) resist the temptation to record lengthy lectures in this format. You may enjoy audiobooks or full-length National Public Radio programs, but recognize that your students may find your very long audio recordings a real struggle to listen through, especially if you lack the time to meticulously edit them like professional productions!
As suggested above, you can have students record themselves. But it's helpful to establish guidelines and even a rubric for such assignments. Insist on a basic level of quality. For example, they should be find a place to record with little background noise, and should expect to edit their files for clarity. Having them write or submit a script beforehand can be part of the assignment. In the script, they might cite sources, and a script to read almost always improves the quality of the recording.