Void Whispers is a short first-person thriller featuring a total of 3 endings and 6 achievements. Since the longest run takes 30 minutes at most, it is recommended to discover the endings by one's self. Once the final scene is reached, the 'Continue' option on the main menu can be used to go for the others. Still, this guide provides clean-cut pointers in case help is needed.
Note that the achievements are tracked in the background and are only granted upon quitting.

Endings are ordered the way they are numbered by the game. Once the final scene is reached:
Broadcast all four messages within the given time limit, leave the station and reach the bunker.
Leave the station without broadcasting the messages and directly go for the bunker.
Do not leave the station until the time runs out, i.e. die trying while trying to broadcast them all.

Two secrets and a platinum.
Before stepping into Cosmo Station, check the opening to the left and interact with the letter.
While seated, grab a paper ball from right, aim as shown above and score -G-. Repeat ten times.
Platinum. Unlocked alongside the third and the final ending the player gets.
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'The universe is a pretty big place. If it's just us, seems like an awful waste of space.' -- Sagan