No.
Although it is not officially supported you could use the .ogg-fileformat instead as it has superior quality at the same filesize.
I would recommend compressing at 128kbit/s with VBR (variable bitrate). This should give you an okayish quality at relatively low filesize.
Also make sure that the file you are compressing actually has good quality and is not some upscaled one.
Read here for more information on audio quality:
t/320007As your map is a marathon you also don't have to worry too much about compression. You have to do one small tweak to your beatmap in order to be allowed to upload a bigger mp3 with your map. That is renaming your sole difficulty to "Marathon". If it is the only difficulty of your map, your beatmapsize can go as far as 100 MB opposed to about 30MB.