You need to obtain a copy of their lease. It's not clear from the attachment.
You can order a copy of her lease from looking up and downloading a copy of the registered title from the land registry's find a property service:-
Once you have a copy of the registered title you should call their customer services number and ask for a copy of the registered lease. They will hold an official copy of it and you can order one for about £11.
The lease itself will have definition of her flat. It will likely be referred to as "Property", "Premises" or "Demised Premises", The definition will describe the actual property she has a lease of in words and will also cross refer to a plan.
If the definition or the plan indicates that the loft space is included then she owns it. If it does not then the loft space will form part of the freehold.
If she does not own it and it forms part of the freehold then she cannot use the area as her own and you will be able to enforce the removal of the speakers, because ownership of it cannot change without a transfer being executed by her and you all together as freeholders (which plainly you would not consent to).
If the area is included in her lease then she is allowed to use it as her own. However, if you consider that she is using the speaker in such a way to cause a nuisance then you can attempt to modify her behaviour by looking for the relevant "NO nuisance or disturbances" clause in the tenant's covenants of her lease. There may also be a covenant in her lease specifically dealing with the times at which she may play music and if so this is another avenue of enforcement for you to use.
Ultimately, you can apply to court for an injunction in the event that you consider that she is breaching the nuisance clauses of her lease.
As to loft spaces generally, it depends from property to property. Some include loft space so that they can sell for a higher price upon original sale, some don't include loft spaces.
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