Go to your 'Team Fortress 2\tf' folder and open 'tf2_sound_misc_dir.vpk' in GCFScape, then go to 'root\sound\music' and there should be 2 files. One should say 'mvm_start_last_wave.wav' and the other one should be 'mvm_end_last_wave.wav'. If you need to know how to modify them, go to your 'Team Fortress 2\tf\custom' folder, then make a new folder and call it whatever you'd like. As an example I'll just say 'last wave sfx'. Once you are in your 'Team Fortress 2\tf\custom\last wave sfx' folder, make a directory called 'sound', and inside sound make a new directory called 'music'. You can drag and drop the files from GCFScape into that folder and modify them with whatever sound program you use (example: Audacity), or just replace them with another sound/song.