The house was built in 1930 and has 9" brick and plaster walls. I have added several sheets of plasterboard and soundproofing to the party wall but would like to add more.
I'm concerned about whether the foundations can handle more weight.
The house party wall is about 6.5 meters high. Would a 5-10% increase in weight pose a problem?

You should have not structural problem but take care making a soundproofing choice. Insulation weights very little and some drywall provideds better sound proofing. QuietRock Soundproof Drywall (http://www.quietsolution.com/html/quietrock.html?utm_source=onthehouse&utm_medium=OTHpartnerbannerQR&utm_campaign=OTH0709)