When I've needed custom audio and compositions, the biggest factors are marketing (I need to be able to find you), existing/prior work, how close the prior work matches the style I'm seeking, and the cost.

On the first one, word of mouth is the best marketing. If I ask online and three of my acquaintances suggest you, I'll consider that strongly.  If I have to search for a composer for the custom work you'll need to be high up in the search results, have a professional online presence, and a good portfolio.  Building a solid network of business relationships takes time, and has its own chicken-and-egg problem: you can't be known until you've done work, and you won't be selected for doing work unless you're known.

On the second one, I need to know you can actually do the job.  If you've got a long list of game credits or have created several professional audio tracks, that shows you can do it. If you've got a big portfolio it suggest you can do it.  If your web site or your discussions don't inspire confidence that you'll do the job, I won't be interested.

On the third one, your portfolio and history need to match.  If I'm looking for someone to compose music for a children's minigame collection I won't hire you if your portfolio is based around steampunk, or around fast-paced music.  However, if I'm working on a game for a steampunk setting, or I'm building a high intensity racing game, those styles will encourage me to use you.

And for money, make it easy to see your price ranges. I need to know that I can afford you. It makes an enormous difference if you're asking for a hundred bucks for some compositions or asking for thousands or even ten thousand dollars.  It doesn't need to be precise, but I should have a good idea before contacting you.