Finding specifications for digital signal processors (DSPs) for given audio application
There are no magic formulas.  However, for a very rough start, find the number of multiply-accumulates (MACs) you need to do per second.  That gives you a lower bound on instruction speed.

To do that, you need to first decide on some parameters:

The sample rate of each signal.

The number of signals you will apply convolutions to.

The width of each convolution in samples.



The total MACs per second is the product of all these.  Figure any competent DSP can do a MAC every cycle once it gets going on a convolution.  Of course there will be extra cycles to start and end each convolution, control overhead, communicating with elsewhere, etc.  You might, for example, start out by reserving 25% of the processor cycles to other than MACs.

Often these processors come in a family of related devices.  One strategy is to prototype with the biggest bestest, then scale down to just what you find you actually need in the production version.  The extra headroom also allows you to do what-if tests in the prototype, add code that might aid in debugging or verification, etc.  It can be very useful to have significant extra headroom in the first prototype.