Вам нужно собрать его вместе, используя alGetBufferi ():
ALint sizeInBytes;
ALint channels;
ALint bits;
alGetBufferi(bufferID, AL_SIZE, &sizeInBytes);
alGetBufferi(bufferID, AL_CHANNELS, &channels);
alGetBufferi(bufferID, AL_BITS, &bits);
lengthInSamples = sizeInBytes * 8 / (channels * bits);
И на длительность в секундах:
ALint frequency;
alGetBufferi(bufferID, AL_FREQUENCY, &frequency);
durationInSeconds = (float)lengthInSamples / (float)frequency;