If a creature token you control that entered this turn is copying something else, then the token enters the battlefield as whatever that creature token copied.

Any enters-the-battlefield abilities of the copied token will trigger when the token enters the battlefield. Any "as [this creature] enters" or "[this creature] enters with" abilities of the copied token will also work.

Each of the tokens created by Redoubled Stormsinger's last ability copies the original characteristics of the token it's copying as stated by the effect that created that token.

Although the tokens are attacking, they were never declared as attacking creatures. This means that abilities that trigger whenever a creature attacks won't trigger when these tokens enter attacking.

You declare which player, planeswalker, or battle each token is attacking as you put it onto the battlefield. It doesn't have to be the same player, planeswalker, or battle as Redoubled Stormsinger or any of the other tokens created by Redoubled Stormsinger's last ability.