Estimate data used on audio-only streams.
Audio-only streaming is roughly two orders of magnitude lighter than video, which is why switching a podcast or music app out of video mode is the single most effective data saving on a phone. At 160 kbps an hour of listening is about 70 MB, so even heavy daily listening rarely troubles a modern data plan. Offline downloads are counted separately because they are one-off transfers rather than continuous streams.
Audio stream data
MB per hour = kbps x 3600 / (8 x 1024); total MB = MB per hour x hours + offline download MB
Spotify streams at 24, 96, 160 or 320 kbps depending on quality setting, Apple Music at 256 kbps AAC, and lossless tiers run 700-1400 kbps. Most spoken-word podcasts are 64-128 kbps mono.
It is roughly four to nine times the data of a 160 kbps stream. On mobile over a capped plan that matters; on home wifi it does not. Whether it is audibly better depends far more on your headphones than on the bitrate.
Because downloads happen once and then cost nothing to replay, while streaming the same track repeatedly re-downloads it every time. The offline share tells you how much of your usage is already optimised.