Commercial CDs sound louder because they are heavily compressed. You can obtain a similar loudness using a compressor plug-in on the master channel, or even two chained compressors with different settings. For example you may first insert a compressor with actual (soft knee) compression settings, and then a compressor with brick-wall-like limiting settings (with the compression curve knee near 0 dB with very low attack and release times). Experiment with this settings until you obtain a good volume without (excessive) clipping.