There are two things you should be concerned about when creating your audio files; those are sampling rate and bit rate.

Sampling rate represents the sound frequency range. The higher sampling rate the file is, the wider the frequency range is. In other words, higher is better quality. Your lows will be lower, your highs will be higher. Lower rates can dampen the highs and lows so that audio quality is lower. You won't notice that much on a cheap set of headphones, but on high quality ones you'll notice a huge difference.

Bit rate defines how many "bits" of space the file takes per second of audio. Obviously the higher the bit rate, the higher quality audio you'll have.

So for best quality, you want high sampling rates and high bit rates. Sampling rates do not affect file size as much as bit rates do, so if space is at a premium (like on an expansion card) a good tradeoff is to create high sampling rate files with a lower bit rate, or use a variable bit rate (VBR) with a moderately high "base" setting, like 128kbps. 

There are a number of ways to tell what rates your files are. The defalut method on Windows based machines is to open the folder they are stored in with Windows Explorer. Hover your mouse over a file (don't click it) and all the file information will pop up in a window including bitrate and sampling rate. Sampling rate is referred to as "Frequency", which makes sense as per my definition above.

If you have files that won't play because of their sampling rate, you can rip them from CD again (if that's how you got them) using a different quality setting, or use an audio conversion program to convert them to a different rate. I like dBpowerAMP available at www.dbpoweramp.com. It's free and very easy to use.