The permanents that entered the battlefield don't need to remain on the battlefield or under your control. Celebration abilities are checking for past events, not the current game state.

Each celebration ability has a different effect.

Raging Battle Mouse can't reduce its own cost, even if it's the second spell you cast in a turn.

Raging Battle Mouse's cost-reduction ability doesn't change the mana cost or mana value of any spell. It changes only the total cost you pay.

Spells that were cast before Raging Battle Mouse entered the battlefield count. If Raging Battle Mouse was the first spell you cast this turn, the next spell you cast this turn is your second spell.

Celebration abilities only care if two or more nonland permanents entered the battlefield under your control in a turn. They won't get more powerful if more than two permanents entered the battlefield under your control in a turn.

Some celebration abilities trigger at specific parts of the turn and check whether two or more nonland permanents entered the battlefield under your control already in that turn.

Others are static abilities that give creatures abilities or power and toughness increases as long as two or more nonland permanents entered the battlefield under your control that turn.

Raging Battle Mouse's cost-reduction ability can't reduce the amount of colored mana you pay for a spell. It reduces only the generic mana component of that cost.