Diddi wrote: they were converted from .mp3 files.
OK, so you already have two levels of quality loss (the quality lost from the original MP3 encoding, then additional loss from the Ogg encoding), and you want to avoid a third layer of quality loss.
Ideally you would go back to the original uncompressed audio (the audio from which the MP3 was made). Amplify that and then export as Ogg (avoiding the MP3 stage altogether).
If you don't have the original uncompressed audio, go back to the MP3.
Unless you specifically need Ogg format, use a lossless editor such as MP3DirectCut (
http://mpesch3.de1.cc/mp3dc.html), or use ReplayGain on your media player (if your media player supports ReplayGain).