The increase is likely because 1.5 Flash was actually cheaper than all other STT services. I wrote about this a while ago at https://ktibow.github.io/blog/geminiaudio/.
I feel that the audio interpreting aspects of the Gemini models aren't just STT. If you give it something like a song, it can give you information about it.
I don't know what they mean by this but the obvious interpretation is not true. It understands other languages, it even does really well with low representation languages, in my case Latvian.