Replacements for mica capacitors?
Don't worry about the internal makeup of the capacitor.  Instead, figure out what you really need the capacitor to do.  Once you have such a spec, then just look around for a cap that meets them.  If you've specified it properly, then you won't really care if the dielectric is mylar, polypropylene, mica, or something else.

The two most important specs are the capacitance and the voltage.  Those alone will push you towards certain technologies.

Since you mentioned audio, you need to get more specific for anything in the signal path.  Many types of ceramic have some piezo effect, so aren't good for audio.  Vibrations and mechanical shock on the device can end up in the signal.  High-capacitance ceramics are also non-linear.

If you only need up to 10 nF or so, but have to stand off high voltage (like in old tube amplifiers), then mylar or even old paper types would work fine.

If this is just to filter out noise, like across a power supply, then the cheaper and smaller ceramics might be fine.

In short, this process starts with good specs.  Without them, you're just poking around in the dark.