I like the “more modern” look.
Regarding the logo, there’s a difficult balance to be struck between “modernizing” and retaining “brand recognition”. The current Audacity logo does perhaps look dated, but it also has terrifically strong association with the “brand”. My personal opinion is that this logo leans a bit too far from the familiar “Audacity brand”.
Regarding the size of the Transport buttons, this is something that has been discussed very recently. While there are arguments to support having “single height” transport buttons, the reason why Audacity has (and will probably retain, at least in the default button set) larger transport buttons is Fitts’s Law.
Nice work joaopauloalbu. I’ll add the logo to the logo gallery