It makes sense for the accounts to be audited in a band, at least internally if not externally. It stops fraud taking place.
If a band is a registered charity, then their accounts need to be in reasonable condition and I am pretty certain this required an external audit.
Note: As for the cost, why not just ask a band friendly accountant to do it as a favour to the band? You don't need to employ the services of a chartered accountancy firm you know!
If the band is not registered as a charity then they are liable to tax. (They are not liable to Corporation Tax however unless the band is a registered limited company.) This is a fact for all bands. I have known bands get a tax return in the past. It can and DOES happen.
Most bands do not have to pay tax however. This is because most bands fall underneath 'non-profit making' organisations. Your accounts can show that whatever money you earn is put back into running the band and purchasing items for the band. No one is making a profit. As tax relates to profits made, no tax needs to be paid. The revenue are within their rights however to inspect/audit the accounts and ensure that no tax evasion is taking place.
I don't think this is set in stone for all brass bands, but it is the generally accepted 'norm'. However, from a safety point of view if you don't have a record of your accounts, then the Revenue could ask you to pay tax on an estimated amoun(by them!) of profit.
If you employ a conductor then they are in theory not only subject to National Insurance but to PAYE as well.
However, as conductors change bands, take more than one band, teach etc etc the conductor is classed as self employed, and they declare their earnings from conducting on a self assessment tax return. If they work in addition to conducting the band, they will not have to pay any NI (you only contribute once!) but will pay tax.
This info is from the remainder of my knowledge from my years in the Chartered world of accountancy so may be not quite as accurate in places as it should be - apologies in advance for this!!