Audio tracks from video files are available for download and can be used in your mixes.
When someone records video in Rumble Studio (on the host or guest side), the audio from their camera/mic is usually recorded too. The only exception would be if they disabled audio recording on their end, which is unlikely.
When you export video files, you have a choice between downloading the original video files (which contain the audio track), or just the audio track taken from the videos (in MP3 / FLAC formats).
If you add videos containing audio to a Mix that is made of a series of audio and video files, and export this Mix as a single merged audio file, then the audio tracks will be taken from the video files and merged with the other audio files. If you choose to download the Mix as a zip containing the original files, then you will receive the video files themselves.
You can test this yourself by recording a short video with some audio, then adding it to a new Mix, then exporting that Mix with different settings (merge vs zip, and mp3 vs original files).