No. Observers are silent Team Members. They cannot voice chat with each other or the Moderator. However Observers can send (text) chat messages to the Moderator and other Team Members who are observing a session. And they can take notes too.
But Observers cannot interact with the Participant, because Participants don't know that Observers are present in the session.
On the other hand, Collaborators who are observing a session can take over Moderation capabilities from the current Moderator and voice chat with the Participant. However we recommend that the original Moderator informs the Participant about switching over to the new Moderator.