The Mazda3 5-door might be a great alternative to the Mazda6. The Mazda3 has 96 cubic feet of interior space compared to the Mazda6's 100 (a very small difference). The MZ3 is also available in higher trim levels with the manual transmission (while only the Mazda6 Sport and Touring can be had with a manual). The MZ3 also weighs about 250lbs less than the larger MZ6, so handling and acceleration would also be a bit better. Both cars can easily hit 60mph in less than 7.5sec, so they're hardly sloiw even if they aren't "fast" like the GTI or Acura TL.
If you're looking for a high-end audio system, the Accord Sport will disappoint...big time. It gets the same 4-speaker 160-watt setup as the Accord LX. Even stepping up to the EX (the highest trim level you can get with the 6-speed manual), it only adds a pair of tweeters to the same system. The EX-L and Touring have much better audio systems, but they're automatic-only.
It doesn't have a lot more power than the Mazdas or the Accord, but the Acura TSX might also work. The driving dynamics and handling are on par with the Mazda3 and Mazda6. It has a 17hp advantage over the Mazdas (201 vs 184) but torque is 15lb-ft less (170 vs. 185). Both the hp and torque peak are also 1000-1300rpm higher than the Mazda. The TSX has never felt underpowered to me (even the heavier Sport Wagon with automatic transmission) and the interior is much nicer than anything you mentioned except the TL.
The TL is a great car to drive, especially with the manual transmission. But I personally can't get past the design...the TSX is much easier on the eyes, IMHO.
But a new Mazda3 Grand Touring 5-door would be at the top of my shopping list...