It can be hard to get ideas for alien or fantasy languages. Even with the four languages I know (and the sound systems of a few more) for ideas, I find myself running dry sometimes. And of course, the last thing you want is for your languages all to sound the same.
Here are a couple of ways to get around this:
One: use a sound system extremely close to an existing world language. The only pitfall here is that your designed language can get a little too close to the language model and people can tell what you're up to. This may not be a problem (many people don't care).
Two: design your sound inventory based on the physiology of your aliens (caterpillar aliens, octopus aliens, cat aliens, etc.) If you've got aliens with a really unusual physiology, this can be fun (if at times difficult to write out using the English writing system).
Sound system itself isn't everything, though. Two languages can have very similar sound systems, but the way the sounds pattern may be entirely different. Japanese and Spanish, for example, are very similar in their sound inventory, but I hardly have to explain that they sound like they come from opposite sides of the planet. They do.
So here's another idea. Listen to animal sounds.
Use the obvious ones, sure, like barking for dogs or meowing for cats. But then take it further. What are the little sounds these animals make? Whining, gurgling, howling, purring, half-meowing, etc. Do they make them repetitively? Try to take these additional sounds and turn them into speech patterns.
Here's an example:
A cat who says "meow" may also say "mrrk" or "meeg", or "mrk-mrk" depending on context. I could imagine that someone who says meow a lot would be fond of diphthongs, so I could say maybe this language doesn't have the lax/short vowels, but only long vowels and diphthongs of all kinds. ow, ai, oi, and push it further. Maybe the length of the vowels would be significant, like a distinction between "meow" and "meeow" or "meoow." Maybe this would be a language where repeating a word indicates a mood of excitement or eagerness.
If I can do this with cat sounds, which are incredibly familiar, imagine what can be done with other animal sounds. There's a ton of stuff out there on the web. Look for monkey sounds, bird sounds, whale sounds, elephant sounds, ground squirrel sounds, anything you like. If you can actually find a recording of it, instead of just imagining it, the task will become even easier and more fun.
Because I'm planning a language design workshop to open on February 1, I thought I'd do a few language-related posts during the month of January. As with the worldbuilding workshop, this one will begin with me asking for submissions, so if you're interested it can't hurt to start thinking now. And listening...