This file is used for identification of PC sounds (in the soundsets, sounds are identified by a letter (male01a, male01b, etc...), but NPC sounds are identified by a number (kivan1, kivan2, etc...). This file convert the numbers to letters.).
The first column indicates the sound slot as defined in SNDSLOT.IDS.
The second one assigns letter used in filenames for players soundset.
In BG1 TotS we have:

2DA V1.0
*
       LETTER
0        *
1        *
2        *
3        *
4        *
5        *
6        b
7        c
8        d
9        a
10       8
11       9
12       *
13       *
14       *
15       *
16       *
17       *
18       !l
19       !m
20       e
21       n
22       o
23       p
24       q
25       r
26       f
27       g
28       h
29       0
30       *
31       *
32       i
33       j
34       k
35       s
36       t
37       u
38       v
39       _
40       *
41       *
42       *
43       *
44       *
45       *
46       *
47       *
48       *
49       *
50       *
51       *
52       *
53       w
54       *
55       *
56       *
57       *
58       *
59       *
60       *
61       *
62       *
63       x
64       y
65       z
66       1
67       2
68       3
69       4
70       5
71       6
72       7