When new user enters room a message is written "User joined room"/"User Se ha unido al chat" - this is a system message that administrator can enable or disable. If it is enabled and you are in this room with checked "Beep"/"Sonido" then you get alerted by sound that new user has entered room.

I think this is what you are looking for, right?

I was talking about the time when somnebody enters the chat, because for us it is not possible to always keep watching the computer screen, but a possible solution could be to warn chatters about inmediately check the small box to activate sounds.
Could it be possible to write a message (in my language, sorry for my bad English)like "Please check the sound box to tell us that you have entered the chat if you need some help about teaching Spanish" ?

Thanks for your fast reply

You will find a little checkbox "Beep" inside the room. This will play sound on incoming message.

We are a public institution and our aim is to train teachers who work teaching spanish to children. Our chat goal is to be consulted by teachers and we need a sound when somebody enters the chat because we are sometimes working a little far from the computer in our office.
Thanks

You can see our chat
www.educa.madrid.org/web/sai.leganes/