You can add audio files you already have available to an editing project by adding them in the Add Media section.
However there's no option to record audio (voice-overs) directly in Clipchamp at the moment, sorry. We are working on this feature and plan to add it soon.
In the mean time, if you have an audio recording application, e.g. most Android or Apple smartphones have one installed, you can record a voice over in one of these, then import the resulting audio file into the Create editor to use in your project (the recording needs to be copied onto your computer first).