To summarize Waylander\'s post, he doubt a sense can be trained if it depends on actual construction of the organ. So some senses can be trained and others not. For attributes, intelligence and will cannot, but charisma and strength can, for example.
Karyuu is not agree for some things. Hellios details how several senses can be trained.
In a nutshell (in real life), all senses depend on your body constitution, as if you have eyeballs to see, you have also ears to hear, etc. You cannot raise your sense beyond what is allowed by your physical constitution: if you have destroyed a part of your ears hearing too loud music for example. If you are myopic, you can gain at most about +0.5 dioptre by excercising your eyes, but that is all, either you are at -1 dioptre or -7 dioptre.
But, even if you cannot improve your senses, you can train them to be more accurate, to use them better. The real truth about this is that an unused (or misused) sense becomes lazy.
So, you can train your hearing to recognize and isolate particular sounds (ex. a man who is an Oreille d\'Or in a submarine (sorry, I haven\'t found the english term)). At home, you can also do blind tests about smells and tastes (or sounds), with small pots containing spices, flowers, anything. Do it regularly with various scents and parfumes, and you will note that your smell has been trained.
For mental and physical attributes, it should be the same. After your body has finished to built itself (when you\'re about 15 years old perhaps), connexions in the brain, skeletton, etc. will not change. So you cannot improve them but you can trained them (body building, training to solve riddles and crosswords, learn to look nice and trustful, etc.)
Now, PS is an imaginary world, so limitations are the ones we (or more likely developpers) impose. If they want all senses to be trainable, they can implement it.
A realistic way to implement it (imho) would be to separate the attribute (your physical and mental constitutions, diminished by wounds and injuries) from the skills.
For example, an Oreille d\'Or has a quite good hearing, but has developped a lot \"recognize sea sounds\" and \"distinguish sounds\" skills.
Also, a swindler may have a quite good Charisma, but has a big score in \"smooth talk\" skill.
Then, the test would be done on (natural_talent + relevant_skill). Il would be much more subtle and full of nuances ...