It is actually fairly simple (look at Korean101 for the specifics here) but keep this in mind: there are absolutely no aspirated/voiceless combinations. They are either aspirated or voiced, NEVER both as it is impossible to do. That said, ㄱㄷㅂㅈ are aspirated (aka voiceless as in k, t, p, and ch (each with hard or medium air release)) OR unaspirated (aka voiced as in g, d, b, and j) depending on where they occur in a word or if they are between a consonant and a vowel or two consonants. Suffice it to say, that the beginning of a word they are aspirated while the middle tend to be voiced, but not always. Take the pure Korean counting: han, too, se, net, tahsot, yahsot, ilgop, yadol, ahop, yul (the first four numbers never pronounce the last letter without a counter). Notice 2 and 8; 2 is with a T (aspirated) and 8 is with a voiced D sound. Also, see the map or Korea: Busan is pronounced Pusan but is now spelled grammatically correct. The others with the lines added are, as said, aspirated, but very, very, little (close the throat and swallow the sound as you try and say the sound) versions of the ones I talked about.