I remember wanting to play Soul Reaver badly but I couldn’t even get past the very beginning because all the instructions were voiced during gameplay.
needswhippedcream, via Reddit

Important speech means that which would make a significant impact to the experience without; either narratively or to someone’s ability to play. In most cases this means speech by protagonists, and instructions and prompts given through speech.

Subtitles are widely relied on by gamers, more than consumers of other types of media. They are used for all kinds of reasons – due to physical hearing loss, due to low quality mobile speakers, due to a noisy environment, because of unpredictable dynamic sounds mixes, to avoid waking the baby, because localisation has been done solely through subtitles, and many other reasons too.

Early in Valve’s Robot Repair demo, I was expected to follow directions I couldn’t hear. I had the sinking realization that VR titles might not include subtitles at all, forcing me to sit out on a lot of opportunities I’m excited to experience.
Karmagon, via Reddit

Subtitles in VR present a design challenge. There is no screen bottom to display them along; floating them at the bottom requires close proximity to the player to avoid occlusion with environmental elements, yet doing so results gives players eye strain from constantly changing the depth they are focusing at; positioning in the environment over players heads has none of these problems but means they will be missed unless the source of the audio happens to be within the players FOV.

However, subtitles in VR are still essential. At time of writing the best solution so far is a hybrid of the above; displayed in the environment while the audio source is within the player’s FOV, and while it is not, floating close to the player with some kind of indication (arrow, left/right alignment etc) of which direction the audio source is located.

Best practice example: Tomb Raider
Best practice example: Bertram Fiddle
Best practice example: Dead to Rights Retribution
Best practice example: Diablo 3

More information: BBC subtitling guidelines