Some software maximises volume by default. An example, compressor compresses encodings in ac3 (dialnorm& film standard compression). Apologies on the terminology error- i don't mean noise reduction (that's usually not in use). I meant compression/normalization (or dialog normalization in the case of dolby). There's usually a checkbox somewhere, or in the settings. Can't seem to find it in Toast, but i remember seeing it in Nero, a couple of years back.
Distortion should not happen on an audio cd, as audio cds allow for dynamic range up to 0dBs without distortion. Run a detect audio peaks and do an audio mixdown before export.
What does it sound like? Can you convert the audio to mp3 and upload it?
Also, 48khz is not audio cd standards, so it would be downsampled to 44.1khz, resulting in some audio problems (but likely not in your case).