Hi @tvwerkhoven,

sorry for completely missing your comment here.
To be honest, I don't have that much experience with electronics (yet 😉), so I cannot really answer confidently.

1. As far as I can see, there's not so much you can tweak on the speaker side. Except for keeping the volume low, of course. So in the end, it's probably the electronics where you can try to reduce battery consumption. With an ESP you definitely have to switch off wifi and bluetooth if you want to get reasonable battery time.
2. I'm not sure what the usual boards offer in that regard. AFAIK there's something like deep sleep which could be worth investigating.
3. The Lilypad has 5 GPIOs on the outer edge. And then there are 6 additional headers in the center, normally used by the rotary encoder. Check out: https://learn.sparkfun.com/tutorials/getting-started-with-the-lilypad-mp3-player#hardware-details-and-hacking-tips. These center pins have hard-wired resistors, so you have to try if they fit your purpose. I used 3 of them for volume and fast-forward/rewind. So if you drop FF/REW, then you should have enough pins for 9 buttons.