Velocity or Capacity

In my opinion neither of them will are right.  I prefer to look at throughput and lead time.  They are based upon the actual work that was done rather than what work is thought to be possible.  

I recommend that you read the book Actionable Agile Metrics for Predictability by Daniel S. Vacanti. It is a good source of information that I have used successfully with many teams. 

I'm beginning to move away from the idea of "velocity", instead focusing on the team's capacity, or how much work they are able to get done. The estimated capacity for an upcoming Sprint is related to how much time the team has to work on the effort considering their past performance and process changes. However, capacity alone is insufficient and the metrics that Daniel mentioned - throughput and lead time - are also important.

When I think only of Scrum theory, can I use capacity or speed to know the average amount of work done by Sprint?

I'd suggest that it's the Developers who should know this. They should know their capacity or velocity when determining how much work they can take on, and how it is calculated.

Are they working out their velocity as an average of some kind, for example? If so, how is it computed? As a Scrum Master, you should ensure that there is transparency and a shared view over such matters.