For the most part, we make these adjustments subconsciously both when we produce and receive speech. We might replace a phoneme that is hard to say, that is, requires moving an articulator to an extreme position, with a similar sound that requires less movement. We've already seen one of these phonological phenomena, liasons. We link the final sounds of a word to the initial sounds in the following word. Assimilation is another simplification that occurs when we speak at a normal rate.
Sounds take on some characteristics of surrounding sounds.
This often occurs because we cannot move our speech articulators quickly enough. This phonological process is called reduction. In general, you should cue what you say, including these simplifications when you make them. In practice it is difficult to hear exactly what you're saying because your brain is giving your conscious mind the complete word, not the individual sounds. Do the best you can.