documentclass[10pt,a4paper]{article}
renewcommand{sfdefault}{PTSansNarrow-TLF} %Changes the default sans serif font, used for figures.
usepackage{sansmath}
usepackage{pgfplots}
pgfplotsset{compat=1.10}
pgfplotsset{/pgf/number format/assume math mode=true}

begin{document}
    begin{sansmath}
        fontfamily{PTSansNarrow-TLF}{fontsize{11}{11}selectfont
        begin{tikzpicture}
            begin{axis}[xlabel = Period ($pi$), ylabel = Amplitude ($Omega$)]
                addplot {sin(deg(x))};
            end{axis}
        end{tikzpicture}
        }
    end{sansmath}
end{document}

begin{newenvironment}
    begin{axis}[xlabel = Period ($pi$), ylabel = Amplitude ($Omega$)]
        addplot {sin(deg(x))};
    end{axis}
end{newenvironment}