Do a sound check at the beginning of every lesson. Do not assume everybody can hear you just because they could last time. You do not want a student to ask you to repeat all that you said after they finally realised you were talking but they couldn’t hear. If you know from the beginning that some students cannot hear you or others, you can, for example, use local chat more.
Personally, if students cannot hear me due to technical problems, I will try to and write as much as I can into local chat to accommodate them. If, however, I have announced that the course will be in voice but students have not set it up, I do not help them during the lesson.
That’s why having one session before the actual course is a good idea or, at least, sending students to places where they can get help (tutorials). Another option, if you possible, is to have a technical assistant, at least in the first couple of lessons and only at the beginning of a lesson, say the first 15 minutes. Students can then be told to IM the assistant for technical help.