Unless the "old" movie is so old that its copyright has expired (prior to 1926 in the US currently, I believe, the precise rule varies by country) or it entered the public domain in some other way (unusual), it is protected by copyright.
One may use part of a copyrighted work only with permission from the copyright owner, unless an exception to copyright applies. Exceptions to copyright vary significantly from country to country In the US, the primary exception is fair use. This answer describes fair use is some detail. Fair use is defined by 17 USC 107.
Whether something is a fair use is a highly-fact-driven determination., There is no clear bright line. Several factors must be considered, including at least the four= statutory factors. A short except, used in a transformative way, that does not harm the market for the original will often qualify as fair use, but details will matter. There is no specific length that qualifies a sound excerpt as fair use. For text, in the well known case of Harper v. Nation an excerpt of less than 300 words from a 500 page book was judged not to be fair use, because it was held to be "the heart of the work" and because its publication significantly harmed the market for the original (a paying contract was canceled because of the use). In other situations, much larger excepts have been held to be fair use.
In some countries there is an exception to copyright known as "fair dealing" which is somewhat similar to fair use, but rather narrower and more restricted. Other countries have a list of specific exceptions (the law of India has more than 20 different exceptions). News reporting, criticism, commentary, and classroom use are common exceptions. In many countries, sampling is not covered by any exception, and permission is required.
If an exception does not apply, and permission is not obtained, then using copyrighted content is copyright infringement. This is, in most cases, a tort, meaning that the copyright owner may sue the infringer. Such a suit may be brought in any country where infringing works are crested or distributed, and possibly in other countries as well. If the plaintiff (owner) wins, damages will be awarded. The rules for determining damages vary widely between countries, and the specific facts of the case will matter.
To get a reliable opinion on whether a specific use qualifies under an exception to copyright, or requires permission, one would do well to consult a lawyer with copyright expertise. Or one could simply ask for permission from the copyright holder. For a publicly released movie, finding the copyright owner should not usually be too hard. The owner may say yes or no, or ask for a fee or other conditions. If the owner simply ignores a request for permission, that must be taken as a "no". For this kind of use, an owner is free to reject or ignore requests for permission if it so chooses. (In the US, cover versions of songs are subject to a compulsory license under certain conditions.)