The budget has been halved this year even though a third of instruments need urgent repair.
"The person with the best CD player is all too often responsible for music teaching in junior schools," says Brent's head of music services, Paul Fensom.
"Everything is biased towards numeracy and literacy. Music is just not taken seriously, yet it can totally transform kids," he added.
Brent's music budget was cut to pound;57,000 for this financial year and Mr Fensom is now the only full-time member of staff.
He said: "Schools think the instruments are theirs and they keep hold of them. I am aware of 1,400 instruments across the borough and at least a third need repair. Some schools have got pianos that should have been set fire to years ago.
"Very few (junior) schools deal with singing nowadays and kids are very self-conscious when they get to secondary school."
For the first time in years, the Bromley Youth Music Trust, which took over the role of the old music service, has had no money to buy new instruments.
A spokesman said: "Most of our instruments are in pretty good condition but they obviously get worn out. If you are not replacing old with new you will end up with a gap. Things are not as rosy as they used to be financially - but compared to what I know of my colleagues elsewhere we have a lot to be thankful for."
The trust provides extra-curricula teaching with 200 full and part-time staff in 100 schools across the south London borough.
Peter Britton, head of Cambridgeshire's Instrumental Music Agency, said schools were rationed to six woodwind and six brass instruments each.
"Instruments are kept pretty well-maintained. We have a little bit of a replacement budget," he said. The county also hires out less popular instruments such as oboes and double basses at 50 per cent discount - taking the strain off the clarinets and flutes.
Mr Britton said music was in danger of being marginalised. His agency, which co-ordinates music in schools, has an annual budget of pound;1.5 million, 18 per cent coming from the local authority and the rest from schools and parents. This year, the council's contribution fell by 5 per cent.
"Some heads are taking the view that if you have music at all, you can only do it out of hours. There is a shortage of music teachers in primary schools. The national curriculum has made that a greater problem because music is no longer viewed as a priority. It is going to the fringes."
Hertfordshire is facing further cuts of pound;200,000 to its music budget next year, already down from pound;1.7 million in 199798 to pound;1.5 million in 199899.
John Witchell, head of a music service with 700 staff, said: "There comes a point when you cannot cut any more. Music services cannot carry on losing money."