I am also running Linux Mint 13, its built on ubuntu 12.04, so here is how I got sound.
1 you down load the files tomas is recomending in his post about ubuntu 12.04
Index of /dists/squeeze-wx/main
I did use the i386 version since my system is 32bit and I didnt use the headers or dev files.
2 you need to go in to synaptic pakage manager and search for libwxbase2.8,
mark it for removal
and apply, synaptic will tell you this will remove libwxbase2.8 ,libwxgtk2.8,opencpn and in my case filezilla. no problem just remeber the programs you remove and install them later.
3 install the two files you have downloaded using GDebi package installer, you will get a warning about a newer version but go on
4 install opencpn again and sound should work!