Another perspective.
In active circuits, the AC impedance (Z) of the output of a dac, preamp, etc. simply can't be measured with a 'static' tool like a dvm.
It can be calculated, or measured by an 'active' device like the one
@michkhol linked in the previous post.
But the circuit being measured needs to be powered up and so a static, non-dynamic, measurement, like from a dvm, will not give you a realistic reading, even if it gives you a reading at all.
And trying to read the output Z from a balanced circuit, which 'should be' transformer coupled, will also result in a misleading reading by a static tool (dvm).
The main difference as michkhol noted, is the impedance Z of an AC circuit (which is used to deliver music signals) also needs to include capacitance (F) and inductance (H) along with resistance (Ω).
This makes for a much more complex and dynamic measurement.
And while measuring a DC circuit only involves just one factor, Ω, which a dvm can measure (but not on an active circuit), it simply can't provide meaningful results on analog nor digital output circuits while trying to measure it's output Ω nor Z.
JJ