Your 1.5Mbps DTS track is lossy DTS Core 5.1 and can be compared to DTS track on DVD. It is not what we usually regard as HD. DTS-HD MA as found in Blu-ray is lossless and variable up to 24.5Mbps.
If you play a Blu-ray (not your file) or ISO with DTS-HD MA 7.1 on a 5.1 system, the extra channels will be mixed into surround speakers by AV amp. The details are not lost but would in theory be clearer on a 7.1 system.