Hi jup,
The best program for doing this would be WavePad which can perform an automated set of actions across a selection of files. Generally, the best method of altering the output volume of a collection of music is to 'normalize' them. Using WavePad's 'Batch Converter' you would select all the files you want to process, select the type of operation (normalize) and then specify how you want the new files stored.
Before doing this I would recommend experimenting with the best normalize settings first on a couple of files only. Once you're happy with the results then you can apply them to all your files.
You could technically do this in MixPad, although the process would be a bit convoluted as it isn't really designed for batch processing of individual files. You would need to manually load each file separately, apply the normalize process and then output the file.
Regards,
Ben