Yes, "if not" is used correctly here, and I think you understand the meaning properly. This is something of an idiomatic usage of "if not", which can have several subtly different meanings. To read it better here, think of it as "even if not" or "although not" or maybe "but not": "They are a wide audience, even if they are not always a discerning audience" or "They are a wide audience even if not a discerning one."

The ambiguity of "if not" comes from the fact that we sometimes use it to mean "maybe even" and sometimes "although maybe not" - which are certainly related senses, although not identical! I would say that when followed with "always", if not almost always has the sense of "although maybe not", since "maybe even always" doesn't make a lot of sense. For example, "'Cold Mountain' at the Academy: Often stirring, if not always inspired" It doesn't really make sense to think of it as it is even always inspired, but it is often stirring. If it were always inspired, it would hardly only be "often" stirring! This is especially obvious in context when reading the article, which says, "[It] has some stirring stretches of music... But there's also a lot of what sounds like filler."

References:
What does 'if not' mean in the given sentence
Usage of 'if' and 'if not' to mean 'and perhaps even/also'
Ambiguity: "if not"

Yes, "if not" is used correctly here, and I think you understand the meaning properly. This is something of an idiomatic usage of "if not", which can have several subtly different meanings. To read it better here, think of it as "even if not" or "although not" or maybe "but not": "They are a wide audience, even if they are not always a discerning audience" or "They are a wide audience even if not a discerning one."

The ambiguity of "if not" comes from the fact that we sometimes use it to mean "maybe even" and sometimes "although maybe not" - which are certainly related senses, although not identical! I would say that when followed with "always", if not almost always has the sense of "although maybe not", since "maybe even always" doesn't make a lot of sense. For example, "'Cold Mountain' at the Academy: Often stirring, if not always inspired" It doesn't really make sense to think of it as it is even always inspired, but it is often stirring. If it were always inspired, it would hardly only be "often" stirring! This is especially obvious in context when reading the article, which says, "[It] has some stirring stretches of music... But there's also a lot of what sounds like filler."

References:
What does 'if not' mean in the given sentence
Usage of 'if' and 'if not' to mean 'and perhaps even/also'
Ambiguity: "if not"

Yes, "if not" is used correctly here, and I think you understand the meaning properly. This is something of an idiomatic usage of "if not", which can have several subtly different meanings. To read it better here, think of it as "even if not" or "although not" or maybe "but not": "They are a wide audience, even if they are not always a discerning audience" or "They are a wide audience even if not a discerning one."

The ambiguity of "if not" comes from the fact that we sometimes use it to mean "maybe even" and sometimes "although maybe not" - which are certainly related senses, although not identical! I would say that when followed with "always", if not almost always has the sense of "although maybe not", since "maybe even always" doesn't make a lot of sense. For example, "'Cold Mountain' at the Academy: Often stirring, if not always inspired" It doesn't really make sense to think of it as it is even always inspired, but it is often stirring. If it were always inspired, it would hardly only be "often" stirring! This is especially obvious in context when reading the article, which says, "[It] has some stirring stretches of music... But there's also a lot of what sounds like filler."

References:
What does 'if not' mean in the given sentence
Usage of 'if' and 'if not' to mean 'and perhaps even/also'
Ambiguity: "if not"

Yes, "if not" is used correctly here, and I think you understand the meaning properly. This is something of an idiomatic usage of "if not", which can have several subtly different meanings. To read it better here, think of it as "even if not" or "although not" or maybe "but not": "They are a wide audience, even if they are not always a discerning audience" or "They are a wide audience even if not a discerning one."

The ambiguity of "if not" comes from the fact that we sometimes use it to mean "maybe even" and sometimes "although maybe not" - which are certainly related senses, although not identical! I would say that when followed with "always", if not almost always has the sense of "although maybe not", since "maybe even always" doesn't make a lot of sense. For example, "'Cold Mountain' at the Academy: Often stirring, if not always inspired" It doesn't really make sense to think of it as it is even always inspired, but it is often stirring. If it were always inspired, it would hardly only be "often" stirring! This is especially obvious in context when reading the article, which says, "[It] has some stirring stretches of music... But there's also a lot of what sounds like filler."

References:
What does 'if not' mean in the given sentence
Usage of 'if' and 'if not' to mean 'and perhaps even/also'
Ambiguity: "if not"