The very nature of soundproofing requires the room to be airtight. This is bad for any room as ventilation and air change are a must.
What kind of music do you make in there? Banging on drums will produce a lot of sweat, and singing will produce a lot of 'breath' (both containing water)
The foil on the celotex and roof felt will prevent any moisture from escaping, rockwool would have been a better product for soundproofing and is also breathable.
Your vents alone won't help if there is nothing to motivate the air to leave (heating or fans for example)
I would suggest fitting at least one extractor fan, with another passive vent at the opposite end of the room. Ideally you'd fit two fans, one in/one out, so the air is constantly getting changed.
Fitting these will create 'holes' in your soundproofing and also annoy you when trying to play music so they'll either need to be installed in expensive soundproof ducting, or you can use standard fans encased in an mdf box lined with rockwool. A couple of S&P TD160's would do the job.
(very popular with those who like to 'grow their own' )
If the mdf is sagging and discoloured, ripping it out and starting again would be the proper way to proceed, and replacing all that celotex while you're at it.