Some of the features have disappeared

If you have an active subscription (or previously bought some extensions), you can reactivate it: in the app press the "i" shaped button under the list of memos, then press "Subscription", finally press "Restore".

Audio Memos has many built in features which can be extended by buying a subscription. The first time you start Audio Memos, you can be asked if you want to try all the extra features for free. You probably answered yes, and now the trial period is over.