FIRST OF ALL YOU NEED HACKED phone 9.X
1.unrar pack there you will find 101F8763.txt and Startup.mp3
2.put 101F8763 to С:/private/10202be9/101F8763.txt (maybe you will need to create new folder 10202be9)
3.put Startup.mp3 to E:\\Sounds\\Simple\\Startup.mp3
4.that's all now turn off you phone and turn on you will have new start-up sound. you may edit Startup.mp3 file but remember it must not be longer than 10 sec !!!

tested on my N73

if it doesn't work on you phone ,you may change manually
1.go to Z:/private/10202be9/ and copy 101F8763.txt
2.put it С:/private/10202be9/101F8763.txt
3.edit 101F8763.txt
you will find there
[Main] remember only change Z:\\Data\\Startup_tone.aac to E:\\Sounds\\Simple\\Startup.mp3
0x1 int 1 0 cap_rd=alwayspass cap_wr=alwaysfail
0x2 int 6 0 cap_rd=alwayspass cap_wr=alwaysfail
0x3 string "Z:\\Data\\Startup_tone.aac" 0 cap_rd=alwayspass cap_wr=alwaysfail and so on
4.put Startup.mp3 to E:\\Sounds\\Simple\\Startup.mp3

additional info

not only mp3 can be used, you may use acc format too !!!!!