I was also quite horrified to see that MP3 capability disappeared in this new version of Sound Studio, but found an explanation on Felttips FAQ page which can be found at on the Felttip website FAQ page.
"The latest version of Sound Studio wont let me save in MP3. How do I fix this?
Because of Mac App Store regulations, we can no longer link to the LAME framework to allow saving for MP3s. We have kept the ability to save in MP3s in the app downloadable on our Resources page. After you download and launch the app, it will automatically detect that youve purchased it from the Mac App Store and will be activated as the full version. Youll continue to get updates to Sound Studio within this app."
Going to the Resources page referred to in the above paragraph, I then downloaded the unlocked version of Sound Studio and the LAME MP3 Encoder, installed them, and the MP3 capability was back - but the App Store didnt recognize it as having already been paid for. Seeing that the App Store "Buy" button was displaying "Install" made me think it was seeing Sound Studio on my hard drive, even though opening Sound Studio still showed the Try or Buy dialog.
Knowing I had to keep the MP3 version I had downloaded from the Resources page, I duplicated it so that if I re-installed from the App Store again, it wouldnt overwrite it.
Re-installing put the App Store version in a folder without touching my MP3 version or its duplicate - but now the MP3 version from the Resources page is recognized by the App Store and is no longer in trial mode.
Having used Sound Studio for several years, I was determined to get it to work like the older versions. Hopefully, this workaround will work the same way for you.