The language is C++.
Actually, probably the right implementation is to change this line:
to:
#ifndef BOOT_VOLUME
#define BOOT_VOLUME VOLUME
#endif
int32_t volume_ = BOOT_VOLUME;
Although the #ifdef might be better to put somewhere else, like after the CONFIG_TOP include in ProffieOS.ino, that way BOOT_VOLUME could be used anywhere in the code.