Sound can motivate, but may also be very distracting. It should be possible to switch off/on different sounds, especially if the sounds are not essential for progression in the game. Background music is an example of sound which can be "removed".
It may be an advantage to use synthetic speech in games which use a lot of text The games should allow for this functionality, but need not necessarily contain the speech driver itself. It is simpler for many users to use voices they are familiar with. MSSAPI is probably the most relevant interface for Windows based PCs.