"Most" HO policies will cover your records and equipment from fire, all subject to the total "Coverage C - Personal Property" limit shown in the policy declarations, w/out sub-limits. There may be endorsements that expand that limit as well, so review your coverages thoroughly. The policy is a contract and says what it says. They need to see you get the full benefit of the policy terms, and to explain all coverages available to you. Generally, they want to pay you what is owed, no more or no less. Valuation of personal property at "like kind and quality" is a thing, nothing new and has been a generally accepted principle for decades. Are some companies/adjusters better, more skilled, more empathetic than others? Yes. People are people in any job/role/industry. They do not stay awake at night thinking for ways to cheat you. The plaintiffs bar in CA is skilled and aggressive. Carriers are fully aware of this.
If you are being told that your audio equipment is simply "not covered" they need to show you the specific policy language that states this. Best of luck.