This is a really interesting question. All of this really depends on YOUR needs, which we cannot know with the details you've provided. So some guiding questions:
- What will be the sound artist's tasks for working on your game? Will they be implementing the audio itself or just creating the audio?
- What device(s) and platforms will your game be launched on? Any specific needs or requires those devices/platforms have for audio assets?
What audio engine do you have in mind for this game? Middleware option or proprietary? Device specific?
- What KIND of audio do you want for the game? Interactive? Linear? Surround/VR/AR type audio?
- Do you want a sound artist that will do everything (music, sound design, voice over direction/recording/editing, mixing, mastering and implementation)? That's a wide net. Sometimes you're better served getting two people who can divide these tasks up amongst their specialities.
No offense but it's a bit odd that you're coming to a forum asking what skills a sound artist should know for your own game. It's almost as if you're not sure what audio your game needs. Usually once a game engine is selected, many of the questions above are answered, at least to a degree. And if you're unsure, and that's okay, then you need someone to act as a consultant. For example, is your game in pre-production or production? Regarding your question about what software should they know - that's really up to you. Are they going to use source control at all? Or are they just going to throw sound assets over the fence for implementation? If they're just creating audio for you - it almost doesn't matter what software they know because so many audio programs can do many of the same things.
When I'm looking for someone to collaborate with I look for several factors:
- quality of their work: does it match the aesthetic I'm after. This is mostly subjective.
- ease of communication: does the freelancer get back to me in a reasonable amount of time. Do their emails make sense? Does it seem like we can get along? Especially during stressful production periods?
- ability to take and incorporate feedback well: you want someone who's open to revisions and open to making compromises so their audio ends up being the best for the project. Not someone who's going to say "nope, the audio is done - no changes." (I've heard some people say this before!!)
- can they do the tasks I need: this goes to an earlier point. A bonus is if they can do all of these tasks and also fulfill a few other extra tasks that I hadn't considered (i.e. maybe they can also program some, or do some light artwork, can write really good dialog, etc).
- their cost: obviously if I cannot afford to pay their rate, how can I hire them? You need to know your budget and know what a person's cost will be. Are they going to charge hourly or per sound asset that is delivered? What is the payment method? Milestones? Monthly? Etc. Lots of details to work out. Or are they salaried? Will you offer benefits? Etc.
And in the end - the only thing that really matters is that you like the audio this person creates and that it serves your game well. If you meet those two items, you've done a great job.