Remove cppcheck-warnings from sound manager and harmonize coding style.

[src/SoundManager.cpp:59]: (style) The scope of the variable 'psnd' can be reduced.
[src/SoundManager.cpp:42]: (warning) Member variable 'Sound::chunk' is not initialized in the constructor.
[src/SoundManager.cpp:152]: (performance) Prefer prefix ++/-- operators for non-primitive types.

No changes in functionality intended.
Hereby I also notify @hean01 who is the original author of the sound manager.