Credit, where credit is due. The code in the OP came from Audrey in hyperlink name with biblatex authoryear (biblatex 1.4b).
You can get the parentheses of \parencite linked if you modify the command itself and introduce a new macro
\newbibmacro*{parencite}{%
\printtext[citehyperref]{%
\DeclareFieldAlias{citehyperref}{noformat}%
\iffirstcitekey
{\bibopenparen}{}%
\ifnumequal{\value{citecount}}{1}
{\usebibmacro{prenote}}{}%
\usebibmacro{cite}%
\ifnumequal{\value{citecount}}{\value{citetotal}}
{\usebibmacro{postnote}}{}%
\iflastcitekey
{\bibcloseparen}{}}}
\DeclareCiteCommand{\parencite}
{}
{\usebibmacro{citeindex}%
\usebibmacro{parencite}}
{\multicitedelim}
{}
\DeclareMultiCiteCommand{\parencites}{\parencite}{\setunit{\multicitedelim}}
We essentially move the parentheses from the wrapper to the bibmacro itself allowing them to be linked to the correct entry at the cost of having to check whether to print the parentheses all the time.
MWE
\documentclass[12pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[style=authoryear]{biblatex}
\usepackage{hyperref}
\addbibresource{biblatex-examples.bib}
\DeclareFieldFormat{citehyperref}{%
\DeclareFieldAlias{bibhyperref}{noformat}% Avoid nested links
\bibhyperref{#1}}
\DeclareFieldFormat{textcitehyperref}{%
\DeclareFieldAlias{bibhyperref}{noformat}% Avoid nested links
\bibhyperref{%
#1%
\ifbool{cbx:parens}
{\bibcloseparen\global\boolfalse{cbx:parens}}
{}}}
\savebibmacro{cite}
\savebibmacro{textcite}
\renewbibmacro*{cite}{%
\printtext[citehyperref]{%
\restorebibmacro{cite}%
\usebibmacro{cite}}}
\renewbibmacro*{textcite}{%
\ifboolexpr{
( not test {\iffieldundef{prenote}} and
test {\ifnumequal{\value{citecount}}{1}} )
or
( not test {\iffieldundef{postnote}} and
test {\ifnumequal{\value{citecount}}{\value{citetotal}}} )
}
{\DeclareFieldAlias{textcitehyperref}{noformat}}
{}%
\printtext[textcitehyperref]{%
\restorebibmacro{textcite}%
\usebibmacro{textcite}}}
\newbibmacro*{parencite}{%
\printtext[citehyperref]{%
\DeclareFieldAlias{citehyperref}{noformat}%
\iffirstcitekey
{\bibopenparen}{}%
\ifnumequal{\value{citecount}}{1}
{\usebibmacro{prenote}}{}%
\usebibmacro{cite}%
\ifnumequal{\value{citecount}}{\value{citetotal}}
{\usebibmacro{postnote}}{}%
\iflastcitekey
{\bibcloseparen}{}}}
\DeclareCiteCommand{\parencite}
{}
{\usebibmacro{citeindex}%
\usebibmacro{parencite}}
{\multicitedelim}
{}
\DeclareMultiCiteCommand{\parencites}{\parencite}{\setunit{\multicitedelim}}
\begin{document}
\cite{sigfridsson} and \parencite{sigfridsson} and \textcite{sigfridsson}
\cite{geer,worman} and \parencite{geer,worman} and \textcite{geer,worman}
\cite{knuth:ct:a,knuth:ct:b,knuth:ct:c} and \parencite{knuth:ct:a,knuth:ct:b,knuth:ct:c} and \textcite{knuth:ct:a,knuth:ct:b,knuth:ct:c}
\parencites{geer}{worman} and \parencites{knuth:ct:a}{knuth:ct:b}{knuth:ct:c}
\printbibliography
\end{document}

\parencite? Yes, the parentheses are not linked there, but that would get more complicated, plus you would have to split the links if you had two citations in the same bracket group. – moewe Mar 15 '16 at 06:56\parecite. – moewe Mar 15 '16 at 14:34