This file assigns the MUS file to certain values and is used by scripting and ARE files to determine what music should be played.
The first column is filled by numbers used by ARE files song sections and scripting commands like PlaySong().
The second one contains of descriptions /identifiers (I have no idea if they can be used instead of values in scripts).
And third one contains of MUS files references that describes what ACM sound files are used and how to play song.

In PSTEE we have:

2DA V1.0
****
            NAME        RESOURCE
0           No_music    ****
1           MAIN        MAIN.mus
2           CHAR_01     CHAR_01.mus
3           CHAR_02     CHAR_02.mus
4           CHAR_03     CHAR_03.mus
5           CHAR_04     CHAR_04.mus
6           CHAR_05     CHAR_05.mus
7           CHAR_06     CHAR_06.mus
8           CHAR_07     CHAR_07.mus
9           CHAR_09     CHAR_09.mus
10          CHAR_10     CHAR_10.mus
11          CHAR_11     CHAR_01.mus
12          CHAR_12     CHAR_12.mus
13          CHAR_13     CHAR_13.mus
14          CHAR_14     CHAR_14.mus
15          CHAR_15     CHAR_15.mus
16          MORT        MORT.mus
17          SIG         SIG.mus
18          SMOL        SMOL.mus
19          BONE        BONE.mus
20          CIVIC       CIVIC.mus
21          RAVE        RAVE.mus
22          MODR        MODR.mus
23          CUR         CUR.mus
24          BAAT        BAAT.mus
25          FORT        FORT.mus
26          CHAR13b     CHAR13b.mus
27          BT1         BT1.mus
28          BT2         BT2.mus
29          BT3         BT3.mus
30          BT4         BT4.mus
31          BT5         BT5.mus
32          BT6         BT5.mus
33          END_01G     END_01G.mus
34          END_01B     END_01B.mus
35          END_02      END_02.mus