This website and podcast offer theme-based, progressive online audio lessons. There are transcripts, worksheets, mobile apps, videos and more. Not all of this content is free, but the beginning of the course is, and it's also available on most podcast platforms, where it's called "Learn Mandarin Chinese - Chinese Audio Lessons"

While this audio course does contain English, it does work in an audio-only format for complete beginners. The host repeats words and phrases over and over, and also includes other voices in the lessons so you can hear Mandarin pronounced by different people (the hos is a foreigner, but his pronunciation is very good). If you're not a complete beginner, you should look for immersive listening resources instead, but this can serve as a good and somewhat safe starting point.

Pros: Easy to understand in audio-only format, progressive lessons, lots of built-in repetition

Cons: Some English translation

This resource was featured in my article The best podcasts for learning Chinese.