N80 is a S60 3rd Edition (initial release) device and 6680 is a S60 2nd Edition, Feature Pack 2 device.
Mixing support was introduced in S60 3rd Edition. In older S60 Editions only one player was able to play at a time. With the MMAPI this is expressed by a notification that mixing is not supported. The
support can be checked from the system property supports.mixing, which returns a boolean value
true if the mixing is supported. When mixing is not supported and two players are started sequentially,
the last call for the start method will cause the first player to stop and the second to start (not in the
Series 40 platform).