Thanks for the tip!
I installed it from Ubuntu Software Center and this problem was solved.
However, now I am getting errors in a lot of functions...
obj/Debug/audio.o||In function `carregar(char*)':|
/home/user/Desktop/Backup/Projects/Project1/audio.cpp|41|undefined reference to `sf::SoundBuffer::LoadFromFile(std::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'|
obj/Debug/audio.o||In function `tocar(int, float, float, bool)':|
/home/user/Desktop/Backup/Projects/Project1/audio.cpp|72|undefined reference to `sf::Sound::SetBuffer(sf::SoundBuffer const&)'|
/home/user/Desktop/Backup/Projects/Project1/audio.cpp|73|undefined reference to `sf::Sound::SetVolume(float)'|
/home/user/Desktop/Backup/Projects/Project1/audio.cpp|74|undefined reference to `sf::Sound::SetPitch(float)'|
/home/user/Desktop/Backup/Projects/Project1/audio.cpp|75|undefined reference to `sf::Sound::SetLoop(bool)'|
/home/user/Desktop/Backup/Projects/Project1/audio.cpp|76|undefined reference to `sf::Sound::Play()'|
obj/Debug/audio.o||In function `tocar(int)':|
/home/user/Desktop/Backup/Projects/Project1/audio.cpp|97|undefined reference to `sf::Sound::SetBuffer(sf::SoundBuffer const&)'|
/home/user/Desktop/Backup/Projects/Project1/audio.cpp|98|undefined reference to `sf::Sound::SetVolume(float)'|
/home/user/Desktop/Backup/Projects/Project1/audio.cpp|99|undefined reference to `sf::Sound::SetPitch(float)'|
/home/user/Desktop/Backup/Projects/Project1/audio.cpp|100|undefined reference to `sf::Sound::SetLoop(bool)'|
/home/user/Desktop/Backup/Projects/Project1/audio.cpp|101|undefined reference to `sf::Sound::Play()'|
obj/Debug/audio.o||In function `tocar_instancia(int)':|
/home/user/Desktop/Backup/Projects/Project1/audio.cpp|113|undefined reference to `sf::Sound::Play()'|
obj/Debug/audio.o||In function `tocar_instancias(int)':|
/home/user/Desktop/Backup/Projects/Project1/audio.cpp|127|undefined reference to `sf::Sound::Play()'|
obj/Debug/audio.o||In function `pausar(int)':|
/home/user/Desktop/Backup/Projects/Project1/audio.cpp|142|undefined reference to `sf::Sound::Pause()'|
obj/Debug/audio.o||In function `pausar_instancias(int)':|
/home/user/Desktop/Backup/Projects/Project1/audio.cpp|157|undefined reference to `sf::Sound::Pause()'|
obj/Debug/audio.o||In function `parar(int)':|
/home/user/Desktop/Backup/Projects/Project1/audio.cpp|172|undefined reference to `sf::Sound::Stop()'|
obj/Debug/audio.o||In function `parar_instancias(int)':|
/home/user/Desktop/Backup/Projects/Project1/audio.cpp|188|undefined reference to `sf::Sound::Stop()'|
obj/Debug/audio.o||In function `ajustar(int, float, float)':|
/home/user/Desktop/Backup/Projects/Project1/audio.cpp|205|undefined reference to `sf::Sound::SetVolume(float)'|
/home/user/Desktop/Backup/Projects/Project1/audio.cpp|208|undefined reference to `sf::Sound::SetPitch(float)'|
obj/Debug/audio.o||In function `ajustar_instancias(int, float, float)':|
/home/user/Desktop/Backup/Projects/Project1/audio.cpp|225|undefined reference to `sf::Sound::SetVolume(float)'|
/home/user/Desktop/Backup/Projects/Project1/audio.cpp|228|undefined reference to `sf::Sound::SetPitch(float)'|
obj/Debug/audio.o||In function `estado(int)':|
/home/user/Desktop/Backup/Projects/Project1/audio.cpp|254|undefined reference to `sf::Sound::GetStatus() const'|
/home/user/Desktop/Backup/Projects/Project1/audio.cpp|256|undefined reference to `sf::Sound::GetStatus() const'|
obj/Debug/audio.o||In function `atualizar()':|
/home/user/Desktop/Backup/Projects/Project1/audio.cpp|286|undefined reference to `sf::Sound::GetStatus() const'|
obj/Debug/audio.o||In function `audio_buffer':|
/home/user/Desktop/Backup/Projects/Project1/audio.hpp|4|undefined reference to `sf::SoundBuffer::SoundBuffer()'|
obj/Debug/audio.o||In function `audio_audio':|
/home/user/Desktop/Backup/Projects/Project1/audio.hpp|10|undefined reference to `sf::Sound::Sound()'|
||=== Build finished: 26 errors, 0 warnings ===|