Is the word voltarem the same in Brazilian and European Portuguese?

Yes

The word "voltarem" is identical in meaning, grammar, and spelling in both Brazilian and Continental Portuguese. It is the third-person plural form of the inflected infinitive or the future subjunctive of the verb voltar (to return/to go back). The only difference is in pronunciation: in Brazil, the vowels are typically more open and clearly articulated, whereas in Portugal, the unstressed vowels undergo "vowel reduction," making the word sound more clipped or "closed."

Brazilian Portuguese Examples

1. Quando eles voltarem, me avise. (When they return, let me/me let know.)
2. Para eles voltarem hoje, precisam de passagens. (For them to return today, they need tickets.)
3. Se eles voltarem para o Brasil, vão ficar felizes. (If they return to Brazil, they will be happy.)
4. É bom eles voltarem logo. (It is good for them to return soon.)
5. Assim que eles voltarem da viagem, eu os vejo. (As soon as they return from the trip, I will see them.)

Continental Portuguese Examples

1. Quando eles voltarem, avise-me. (When they return, let me know.)
2. Para eles voltarem hoje, precisam de bilhetes. (For them to return today, they need tickets.)
3. Se eles voltarem para o Brasil, ficarão contentes. (If they return to Brazil, they will be happy.)
4. É bom eles voltarem logo. (It is good for them to return soon.)
5. Ao voltarem da viagem, eles descansarão. (Upon returning from the trip, they will rest.)