Traffic to Lambda from the internet and between Lambda and S3 is free. The only thing you pay for are the transfer costs from S3 (at cents per GB).

Assuming resulting audio of 3 minutes, then 1000 uses would result in 9 GB, or about 81 cents. As long as you can get ads for $1 per mille, you should be good. That said, you'd probably need to implement something to prevent abuse (single user bypassing the frontend and just spamming your backend).

Looking forward to the next post in the series.

What if you use an S3 upload hook? Are you charged for bandwidth S3 -> lambda? Also, wouldn't you use the same bandwidth with elastic transcoder anyway?