The answer is, "That depends." You may have a reasonable expectation of privacy in your conversations. A reasonable expectation of privacy may exist in a person's home, but statements made in the presence of "outsiders" are not protected since the statements are in "plain view" with no intention to keep the matter private. Further, conversations held out in the open that are not protected from being heard would not be considered private.
With most telephone conversations, there is a reasonable expectation of privacy (although a possible exception to this is when a public telephone is used).
Secretly recorded conversations require further analysis. Federal law prohibits the intentional recording of oral communications under most circumstances. One notable exception to the federal law is when the person making the recording is a party to the conversation, or has consented to the recording (so long as the recording is not for the purpose of committing any criminal or tortuous act). Thus under federal law it may be permissible for a person to record his/her conversations with others and then use the recording as evidence.
Some states have laws which differ from the federal approach. For example, in the State of California it is a crime to secretly record a conversation (unless all parties to the conversation consent to its recording). Law enforcement officers are entitled to record conversations under both federal and state law when certain conditions are met.