Teo Xypolitidis, Below are links 1st is tear down to access speaker and 2nd link is a replacement speaker, you can also do a google search for more options on getting a replacement speaker. Good luck.
I hope this helped you out, if so let me know by pressing the helpful button.
[guide|32575]
http://www.ebay.com/itm/OEM-UNLOCKED-CATERPILLAR-CAT-B15-REPLACEMENT-LOUDSPEAKER-LOUD-SPEAKER-/162115817618