Duke3D: Sounds defined in DEF are inaudible with a volume of 1.0

In commit r8291 (501f8e67) volume control was altered such that it now uses fix16_t instead of float.

This commit forgot to change the sound DEF command, which uses a double type variable to store the sound volume. This is then passed to S_DefineSound which implicitly converts it to float, and it then attempts to convert the value from float to fix16_t.

The assumption made in the DEF sound command is that the default volume is 1.0. However, since the internal default volume with fix16_t is 0x00010000, the implicit conversion results in a fix16_t value of 0x00000001, which is completely inaudible. In fact, setting the volume to 65536 inside the DEF script results in the normal volume level, confirming my theory.