Volume is an issue caused by original sharer. As you may or may not be aware, not all CD music has music at same volume levels. Also, many mp3 id3 tags allow you to artificially adjust the volume to maximize it. Check the id3 tag info to see if you can adjust the sound levels so all those you wish to keep together are similar.

If the desire is to burn to audio CD, then you can normalise the volume of each song so they are all at maximum possible volume. Though wiser is to maximise the ones that are obviously at an unnatural lower volume all together. Then maximise those with a higher volume & obviously from same source together. Then compare to see if they all listen together properly. To maximise music files they should first be converted to a non-lossy format such as WAV or AIFF, then maximised using whatever audio utility you may have. (re-saving files as mp3 will result in loss of sound quality, which is why they should be converted to wav before burning if pre-burning processing is desired.)