The quality of audio from the pre-recorded prompts is based on so many factors it’s hard to determine if a single action (say, installing a Digium card) would address your concerns.
The quality of the microphone/studio used to record the prompts.
The sample rate of the prompt recordings.
The compression rate of the file.
The ability of the DSP or processor to keep up with the playback.
The quality of the phone or speaker used to hear and compare the two.
Just saying, “buy a Digium card” is, I think, a little simplistic. The first thing to do is be certain all things are equal.
You should know the sample rate and compression of both files you are playing, remembering that a severe compression rate and a low sample rate would degrade sound on either system.
You should remember that if you test a Digium card with a single port in service the processor may keep up nicely, but may stutter and fail with all ports in service.
Engineering systems like this is always a balancing act. Yes, it’s nice to have the best sounding prompts in the business, and I think it’s good that you take the time to compare things like that.
However, if you have to buy the most expensive boards and have huge storage for prompt files it increases your cost to deploy a system.